selinux: Convert isec->lock into a spinlock

Convert isec->lock from a mutex into a spinlock.  Instead of holding
the lock while sleeping in inode_doinit_with_dentry, set
isec->initialized to LABEL_PENDING and release the lock.  Then, when
the sid has been determined, re-acquire the lock.  If isec->initialized
is still set to LABEL_PENDING, set isec->sid; otherwise, the sid has
been set by another task (LABEL_INITIALIZED) or invalidated
(LABEL_INVALID) in the meantime.

This fixes a deadlock on gfs2 where

 * one task is in inode_doinit_with_dentry -> gfs2_getxattr, holds
   isec->lock, and tries to acquire the inode's glock, and

 * another task is in do_xmote -> inode_go_inval ->
   selinux_inode_invalidate_secctx, holds the inode's glock, and
   tries to acquire isec->lock.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
[PM: minor tweaks to keep checkpatch.pl happy]
Signed-off-by: Paul Moore <paul@paul-moore.com>