mm: only make map_swap_entry available for CONFIG_HIBERNATION

Current tree spews this on compile:

mm/swapfile.c:2290:17: warning: ‘map_swap_entry’ defined but not used [-Wunused-function]
 2290 | static sector_t map_swap_entry(swp_entry_t entry, struct block_device **bdev)
       |                 ^~~~~~~~~~~~~~

if !CONFIG_HIBERNATION, as we don't use the function unless we have that
config option set.

Fixes: 48d15436 ("mm: remove get_swap_bio")
Signed-off-by: Jens Axboe <axboe@kernel.dk>