allow posix_fallocate in capability mode

posix_fallocate is logically equivalent to writing zero blocks to the
desired file size and there is no reason to prevent calling it in
capability mode. posix_fallocate already checked for the CAP_WRITE
right, so we merely need to list it in capabilities.conf.

Reviewed by: allanjude
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D12640