rpc_pipe: saner primitive for creating subdirectories

rpc_pipe: saner primitive for creating subdirectories

All users of __rpc_mkdir() have the same form - start_creating(),
followed, in case of success, by __rpc_mkdir() and unlocking parent.

Combine that into a single helper, expanding __rpc_mkdir() into it,
along with the call of __rpc_create_common() in it.

Don't mess with d_drop() + d_add() - just d_instantiate() and be
done with that.  The reason __rpc_create_common() goes for that
dance is that dentry it gets might or might not be hashed; here
we know it's hashed.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>