Fix enforcement of file size limit with O_APPEND on ZFS.

vn_rlimit_fsize takes uio->uio_offset and uio->uio_resid into account
when determining whether given write would exceed RLIMIT_FSIZE.

When APPEND flag is specified, ZFS updates uio->uio_offset to point to the
end of file.

But this happens after a call to vn_rlimit_fsize, so vn_rlimit_fsize check
can be rendered ineffective by thread that opens some file with O_APPEND
and lseeks below RLIMIT_FSIZE before calling write.

Submitted by: Mateusz Guzik <mjguzik at gmail dot com>
MFC after: 2 weeks