xfs: turn the xfs_trans_mod_dquot_byino stub into an inline function

Without this upcoming change can cause an unused variable warning,
when adding a local variable for the fields field passed to it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>