vfs: compare the maximum file size unsigned in vn_rlimit_fsizex()

[nprice]

Summary

uio_offset + uio_resid wraps for an offset near OFF_MAX and passes the
maximum file size check. nfsd writes skip RLIMIT_FSIZE, so a WRITE at
OFF_MAX reached ffs_write() and grew the file's last fragment before
failing with EFBIG. A local pwrite() at OFF_MAX now fails with EFBIG
instead of being killed by SIGXFSZ.