Don't leak lock from os_kmem_alloc().

The current port patches for os_kmem_alloc() add VM object locking.
However, the lock is not unlocked in the success case because the
unlock code is inside of an if body instead of after the if statement.

Reported by: jpaetzel
Reviewed by: jpaetzel
Approved by: swills
MFH: 2017Q3
Differential Revision: https://reviews.freebsd.org/D12038