An unconfirmed claim by Tolkien gateway suggests that they are inspirations from real-world surnames, likely the same for C.S. Forester, but here's both sides of the coin.
C.S. Forester's Hornblower's first appearance was in February of 1937. The same year Tolkien began work on his sequel to the Hobbit, which would come to be known as The Lord of the Rings. Tolkien, however, started 7-9 months later, some time in September-November 1937
21 September 1937: The Hobbit is published. Because of its success, Stanley Unwin subsequently urges Tolkien to write a sequel, which he begins. This is the germ of The Lord of the Rings.
15 Nov-19 Dec 1937: Tolkien resumes work on the Quenta Silmarillion, but abandons this to work on ‘The new Hobbit’.
The Tolkien Society's timeline for J.R.R. Tolkien
Tolkien had reached the introduction of the Black Riders by March of that next year, suggesting he had the first chapters down quite quickly. Considering he was in the process of publishing the Hobbit and getting his other works together (Such as the Quenta and Beowulf: The Monsters and The Critics) it's possible this novel by C.S. Forester had simply gone unnoticed and Tolkien took the names from common family names that suited his Hobbits.
According to the Lord of the Rings - A Reader's Companion, Tolkien states the following in his "Nomenclature of the Lord of the Rings"
Bracegirdle. A genuine English surname. Used in the text, of course, with reference to the hobbit tendency to be fat and so to strain their belts. A desirable translation would recognize this by some equivalent meaning: Tight-belt, or Belt-tightener/strainer/stretcher. (The name is a genuine English one; a compound of the Romance type with verbal element first, as Drinkwater — Boileau; but it is not necessary that the representation should be a known surname in the language of translation.)
Buckland is also meant to contain the same animal name (Ger. Bock), though Buckland, an English place-name, is frequently in fact derived from 'book-land', land originally held by a written charter.
Hornblower. Hornblow, -er are E[nglish] surnames, in the Shire evidently occupational surnames. Translate by sense.
The Lord of the Rings: A Reader's Companion - Nomenclature of the Lord of the Rings, by J.R.R. Tolkien
Yes, these are reasonably common surnames in Britain and its Empire. Hornblower has 9 entries on Wikipedia, Bracegirdle (which has the fewest but has the Bracegirdle affair) has 6 entries and Buckland has 23 entries. On ancestry.com the surname with the least mentions was the Hornblowers with a whopping ~4000 entries, followed by the Bracegirdles which had ~8000 entries and finally the Bucklands with ~115000 entries. The histories of each of the "houses" can also be found on houseofnames.com. (Hornblower, Bracegirdle and Buckland)
TolkienGateway lists C.S. Forester as the inspiration for the name Bracegirdle as well as Hornblower.
The name Bracegirdle of a character in the Horatio Hornblower novels by C.S. Forester
See #Hornblower (links to nowhere)
TolkienGateway: Tolkien's inspirations, Bracegirdle