add a check for filesize rlimit to nfs_allocate

[rmacklem]

Summary

As reported in PR#260343, nfs_allocate() did not check
the filesize rlimit. This patch adds that check.

Test Plan

Tested against a NFSv4.2 server exporting a UFS file
system with Allocate enabled.

Doing posix_fallocate() was tested when both the
limit would not be exceeded and the filesize limit
would be exceeded.