eCryptfs: Revert "dont call lookup_one_len to avoid NULL nameidata"

eCryptfs: Revert "dont call lookup_one_len to avoid NULL nameidata"

This reverts commit b0aeaa5c5188ab7515fe2e996b331ccad12e11e3 and commit
22345c4a832cd157540cf4e781ab0a1a0eb45e15, which fixed a regression by
the former.

Al Viro pointed out bypassed dcache lookups in
ecryptfs_new_lower_dentry(), misuse of vfs_path_lookup() in
ecryptfs_lookup_one_lower() and a dislike of passing nameidata to the
lower filesystem.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>