NFS: Release dcache_lock in an error path of nfs_path

In one of the error paths of nfs_path, it may return with dcache_lock still
held; fix this by adding and using a new error path Elong_unlock which unlocks
dcache_lock.

Signed-off-by: Josh Triplett <josh@freedesktop.org>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
(cherry picked from f4b90b43677fb23297c56802c3056fc304f988d9 commit)