Remove an unnecessary memory barrier (implicit in rcu_dereference()) from
install_session_keyring().

install_session_keyring() is also rearranged a little to make it slightly
more efficient.

As install_*_keyring() may schedule (in synchronize_rcu() or
keyring_alloc()), they may not be entered with interrupts disabled - and so
there's no point saving the interrupt disablement state over the critical
section.

exec_keys() will also be invoked with interrupts enabled, and so that doesn't
need to save the interrupt state either.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>