coda: BKL ioctl pushdown

Convert coda_pioctl to an unlocked_ioctl pushing down the BKL
into it.

Signed-off-by: John Kacur <jkacur@redhat.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jan Harkes <jaharkes@cs.cmu.edu>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>