frontswap: make frontswap_init use a pointer for the ops

frontswap: make frontswap_init use a pointer for the ops

This simplifies the code in the frontswap - we can get rid of the
'backend_registered' test and instead check against frontswap_ops.

[v1: Rebase on top of 703ba7fe5e0 (ramster->zcache move]
Signed-off-by: Konrad Rzeszutek Wilk <[email protected]>
Signed-off-by: Bob Liu <[email protected]>
Cc: Wanpeng Li <[email protected]>
Cc: Andor Daam <[email protected]>
Cc: Dan Magenheimer <[email protected]>
Cc: Florian Schmaus <[email protected]>
Cc: Minchan Kim <[email protected]>
Cc: Stefan Hengelein <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>