dquot: cleanup dquot transfer routine

Get rid of the transfer dquot operation - it is now always called from
the filesystem and if a filesystem really needs it's own (which none
currently does) it can just call into it's own routine directly.

Rename the now static low-level dquot_transfer helper to __dquot_transfer
and vfs_dq_transfer to dquot_transfer to have a consistent namespace,
and make the new dquot_transfer return a normal negative errno value
which all callers expect.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext4/super.c

@@ -1017,7 +1017,6 @@ static const struct dquot_operations ext4_quota_operations = {
 #ifdef CONFIG_QUOTA
 	.get_reserved_space = ext4_get_reserved_space,
 #endif
-	.transfer	= dquot_transfer,
 	.write_dquot	= ext4_write_dquot,
 	.acquire_dquot	= ext4_acquire_dquot,
 	.release_dquot	= ext4_release_dquot,