quota: Fix dquot_transfer for filesystems different from ext4

commit 05b5d898 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

fs/quota/dquot.c

@@ -1425,6 +1425,9 @@ 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);