This patch removes the security_inode_post_lookup hook entirely and
adds a security_d_instantiate hook call to the d_instantiate function
and the d_splice_alias function.  The inode_post_lookup hook was
subject to races since the inode is already accessible through the
dcache before it is called, didn't handle filesystems that directly
populate the dcache, and wasn't always called in the desired context
(e.g. for pipe, shmem, and devpts inodes).  The d_instantiate hook
enables initialization of the inode security information.  This hook
is used by SELinux and by DTE to setup the inode security state, and
eliminated the need for the inode_precondition function in SELinux.