compat_ioctl: inline all conversion handlers

This makes all ioctl conversion handlers called from
a single switch statement, leaving only COMPATIBLE_IOCTL
and ULONG_IOCTL statements in the table. This is somewhat
more space efficient and also lets us simplify the
handling of the lookup table significantly.

before:
   text    data     bss     dec     hex filename
   7619   14024    2080   23723    5cab obj/fs/compat_ioctl.o
after:
   7567   13352    2080   22999    59d7 obj/fs/compat_ioctl.o

Signed-off-by: Arnd Bergmann <arnd@arndb.de>