9p: Convert semaphore to spinlock for p9_idpool

When booting from v9fs, down_interruptible in p9_idpool_get() triggered a BUG
as it was being called with IRQs disabled.  A spinlock seems like the right
thing to be using since the idr functions go out of their way not to sleep.

This patch eliminates the BUG by converting the semaphore to a spinlock.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Acked-by: Eric Van Hensbergen <ericvh@gmail.com>