spufs: fix gang directory lifetimes

prior to "[POWERPC] spufs: Fix gang destroy leaks" we used to have
a problem with gang lifetimes - creation of a gang returns opened
gang directory, which normally gets removed when that gets closed,
but if somebody has created a context belonging to that gang and
kept it alive until the gang got closed, removal failed and we
ended up with a leak.

Unfortunately, it had been fixed the wrong way.  Dentry of gang
directory was no longer pinned, and rmdir on close was gone.
One problem was that failure of open kept calling simple_rmdir()
as cleanup, which meant an unbalanced dput().  Another bug was
in the success case - gang creation incremented link count on
root directory, but that was no longer undone when gang got
destroyed.

Fix consists of
	* reverting the commit in question
	* adding a counter to gang, protected by ->i_rwsem
of gang directory inode.
	* having it set to 1 at creation time, dropped
in both spufs_dir_close() and spufs_gang_close() and bumped
in spufs_create_context(), provided that it's not 0.
	* using simple_recursive_removal() to take the gang
directory out when counter reaches zero.

Fixes: 877907d3 "[POWERPC] spufs: Fix gang destroy leaks"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>