smbfs: Push down BKL into ioctl function

Converting from ->ioctl to ->unlocked_ioctl with explicit
lock_kernel lets us kill the ioctl operation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[fixed inode reference in smb_ioctl]
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>