[POWERPC] spufs: Fix state_mutex leaks

Fix various state_mutex leaks.  The worst one was introduced by the
interrutible state_mutex conversion but there've been a few before
too.  Notably spufs_wait now returns without the state_mutex held
when returning an error, which actually cleans up some code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Luke Browning <lukebrowning@us.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>