rpc_populate(): lift cleanup into callers

rpc_populate(): lift cleanup into callers

rpc_populate() is called either from fill_super (where we don't
need to remove any files on failure - rpc_kill_sb() will take
them all out anyway) or from rpc_mkdir_populate(), where we need
to remove the directory we'd been trying to populate along with
whatever we'd put into it before we failed.  Simpler to combine
that into simple_recursive_removal() there.

Note that rpc_pipe is overlocking directories quite a bit -
locked parent is no obstacle to finding a child in dcache, so
keeping it locked won't prevent userland observing a partially
built subtree.

All we need is to follow minimal VFS requirements; it's not
as if clients used directory locking for exclusion - tree
changes are serialized, but that's done on ->pipefs_sb_lock.

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