d_hash_and_lookup(): export, switch open-coded instances

* calling conventions change - ERR_PTR() is returned on ->d_hash() errors;
NULL is just for dcache miss now.
* exported, open-coded instances in ncpfs and cifs converted.

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

diff --cc fs/cifs/readdir.c
Simple merge

diff --cc fs/dcache.c
Simple merge

diff --cc fs/ncpfs/dir.c
Simple merge

diff --cc fs/proc/base.c
Simple merge