ipc/sem: drop __sem_free()

The remaining users of __sem_free() can simply call kvfree() instead for
better readability.

[manfred@colorfullife.com: Rediff to keep rcu protection for security_sem_alloc()]
Link: http://lkml.kernel.org/r/20170525185107.12869-20-manfred@colorfullife.com


Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Manfred Spraul <manfred@colorfullife.com>
Cc: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>