SELinux: Institute file_path_has_perm()

Create a file_path_has_perm() function that is like path_has_perm() but
instead takes a file struct that is the source of both the path and the
inode (rather than getting the inode from the dentry in the path).  This
is then used where appropriate.

This will be useful for situations like unionmount where it will be
possible to have an apparently-negative dentry (eg. a fallthrough) that is
open with the file struct pointing to an inode on the lower fs.

Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>