ext4: fix an IS_ERR() vs NULL check

The ext4_find_extent() function never returns NULL, it returns error
pointers.

Fixes: 44059e503b03 ("ext4: fast commit recovery path")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20201023112232.GB282278@mwanda


Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org