Allow posix_fadvise in capability mode

Allow posix_fadvise in capability mode

posix_fadvise operates only on a provided fd.  Noted by
Mathieu <sigsys@gmail.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)