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>