ext4: use IS_ERR() for error checking of path

With this fix, fast commit recovery code uses IS_ERR() for path
returned by ext4_find_extent.

Fixes: 8016e29f

 ("ext4: fast commit recovery path")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Link: https://lore.kernel.org/r/20201027204342.2794949-1-harshadshirwadkar@gmail.com


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