Add vn_rlimit_fsizex() and vn_rlimit_fsizex_res()

Add vn_rlimit_fsizex() and vn_rlimit_fsizex_res()

The vn_rlimit_fsizex() function:
- checks that the write does not exceed RLIMIT_FSIZE limit and fs
  maximum supported file size
- truncates write length if it exceeds the RLIMIT_FSIZE or max file size,
  but there are some bytes to write
- sends SIGXFSZ if RLIMIT_FSIZE would be exceed otherwise

POSIX mandates the truncated write in case when some bytes can be
written but whole write request fails the RLIMIT_FSIZE check.

The function is supposed to be used from VOP_WRITE()s. Due to
pecularity in the VFS generic write syscall layer, uio_resid must
correctly reflect the written amount (noted by markj). Provide the dual
vn_rlimit_fsizex_res() function to correct uio_resid after the clamp
done in vn_rlimit_fsizex() on VOP_WRITE() return.

PR:	164793
Reviewed by:	asomers, jah, markj
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Differential revision:	https://reviews.freebsd.org/D36625