LSM: Pass linux_binprm pointer to open_exec() so creds are available

Pass the exec state (struct linux_binprm) pointer to open_exec() so new creds
are available for a later patch to use.

Also:

 (1) Move the declaration of open_exec() to linux/binfmts.h.

 (2) Stick a kerneldoc banner comment on open_exec() describing it.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Serge Hallyn <serge.hallyn@ubuntu.com>