phantom: move to unlocked_ioctl

phantom's ioctl is often (4000 times a sec or so) invoked, don't acquire
BKL and block other processes.

Signed-off-by: Jiri Slaby <xslaby@phantom.fi.muni.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>