[POWERPC] spufs: add enchanced simple attr macros

The simple attr macros currently used by spufs can't deal with the
handlers returning errors, which is required to make the state_mutex
interruptible.  This adds a local copy that allows for an error
return from the get/set handlers.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>