ext4: change ext4_xattr_inode_iget() signature

In general, kernel functions indicate success/failure through their return
values. This function returns the status as an output parameter and reserves
the return value for the inode. Make it follow the general convention.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>