selinux: shrink critical section in sel_write_load()

selinux: shrink critical section in sel_write_load()

Currently sel_write_load() takes the policy mutex earlier than
necessary. Move the taking of the mutex later. This avoids
holding it unnecessarily across the vmalloc() and copy_from_user()
of the policy data.

Cc: stable@vger.kernel.org
Signed-off-by: Stephen Smalley <stephen.smalley.work@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>