ext2: Fix fs corruption in ext2_get_xip_mem()

Commit 8e3dffc6 "Ext2: mark inode dirty after the function
dquot_free_block_nodirty is called" unveiled a bug in __ext2_get_block()
called from ext2_get_xip_mem(). That function called ext2_get_block()
mistakenly asking it to map 0 blocks while 1 was intended. Before the
above mentioned commit things worked out fine by luck but after that commit
we started returning that we allocated 0 blocks while we in fact
allocated 1 block and thus allocation was looping until all blocks in
the filesystem were exhausted.

Fix the problem by properly asking for one block and also add assertion
in ext2_get_blocks() to catch similar problems.
Reported-and-tested-by: Andiry Xu <andiry.xu@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/inode.c

@@ -632,6 +632,8 @@ static int ext2_get_blocks(struct inode *inode,
 	int count = 0;
 	ext2_fsblk_t first_block = 0;
 
+	BUG_ON(maxblocks == 0);
+
 	depth = ext2_block_to_path(inode,iblock,offsets,&blocks_to_boundary);
 
 	if (depth == 0)

fs/ext2/xip.c

@@ -35,6 +35,7 @@ __ext2_get_block(struct inode *inode, pgoff_t pgoff, int create,
 	int rc;
 
 	memset(&tmp, 0, sizeof(struct buffer_head));
+	tmp.b_size = 1 << inode->i_blkbits;
 	rc = ext2_get_block(inode, pgoff, &tmp, create);
 	*result = tmp.b_blocknr;