Linux: always use afs_maybe_unlock_kernel

Linux: always use afs_maybe_unlock_kernel

In one error case in afs_linux_lookup unlock_kernel() is called
directly instead of using the conditional "maybe" form.
If the config is such that the BKL is not taken, this can result
in an attempt to unlock when the lock has not been taken, and
can cause an oops.

Change-Id: I27d5db58e9b9d45d925b24e27bc43960499c6ead
Reviewed-on: http://gerrit.openafs.org/790
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>