compat: move sockios handling to net/socket.c

This removes the original socket compat_ioctl code
from fs/compat_ioctl.c and converts the code from the copy
in net/socket.c into a single function. We add a few cycles
of runtime to compat_sock_ioctl() with the long switch()
statement, but gain some cycles in return by simplifying
the call chain to get there.

Due to better inlining, save 1.5kb of object size in the
process, and enable further savings:

before:
   text    data     bss     dec     hex filename
  13540   18008    2080   33628    835c obj/fs/compat_ioctl.o
  14565     636      40   15241    3b89 obj/net/socket.o

after:
   text    data     bss     dec     hex filename
   8916   15176    2080   26172    663c obj/fs/compat_ioctl.o
  20725     636      40   21401    5399 obj/net/socket.o

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>