ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable

lower_dentry can't go from positive to negative (we have it pinned),
but it *can* go from negative to positive.  So fetching ->d_inode
into a local variable, doing a blocking allocation, checking that
now ->d_inode is non-NULL and feeding the value we'd fetched
earlier to a function that won't accept NULL is not a good idea.

Cc: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>