xfs: move write verification out of _xfs_buf_ioapply

Split the write verification logic out of _xfs_buf_ioapply into a new
xfs_buf_verify_write helper called by xfs_buf_submit given that it isn't
about applying the I/O and doesn't really fit in with the rest of
_xfs_buf_ioapply.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Acked-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>