quota: Fix dquot_transfer for filesystems different from ext4

Commit fd8fbfc1 modified the way we find amount of reserved space
belonging to an inode. The amount of reserved space is checked
from dquot_transfer and thus inode_reserved_space gets called
even for filesystems that don't provide get_reserved_space callback
which results in a BUG.

Fix the problem by checking get_reserved_space callback and return 0 if
the filesystem does not provide it.

CC: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/dquot.c

@@ -1377,6 +1377,9 @@ static void inode_sub_rsv_space(struct inode *inode, qsize_t number)
 static qsize_t inode_get_rsv_space(struct inode *inode)
 {
 	qsize_t ret;
+
+	if (!inode->i_sb->dq_op->get_reserved_space)
+		return 0;
 	spin_lock(&inode->i_lock);
 	ret = *inode_reserved_space(inode);
 	spin_unlock(&inode->i_lock);