posixshm: implement posix_fallocate(2)

Linux expects to be able to use posix_fallocate(2) on a memfd. Other places
would use this with shm_open(2) to act as a smarter ftruncate(2).

Test has been added to go along with this.

Reviewed by: kib (earlier version)
Differential Revision: https://reviews.freebsd.org/D23042