Allow posix_fadvise in capability mode
posix_fadvise operates only on a provided fd. Noted by
Mathieu <email@example.com> in review D34761.
No new CAP_ rights are added for posix_fadvise(), as 'advice' in
general only influences when I/O happens; the fd must have existing
CAP_ rights for actual data access.
Reviewed by: markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34903
(cherry picked from commit 52a1d90c8bfe80a15e71a1a2bd7b9407949741d6)