extattr_get_fd(2): fix manlint errors

• The CAVEATS section was misspelled as "CAVEAT".
• The CAVEATS section should come before the "BUGS" section and after other existing sections by convention.

MFC after: 1 week
Reported by: make manlint
Sponsored by: DellEMC Isilon

(cherry picked from commit 9d610b15166d51a4ca0f96237ed71f6edb833434)