check_swap_activate() will lookup block mapping via bmap() one by one, so
its performance is very bad, this patch introduces check_swap_activate_fast()
to use f2fs_fiemap() to boost this process, since f2fs_fiemap() will lookup
block mappings in batch, therefore, it can improve swapon()'s performance
significantly.

Note that this enhancement only works when page size is equal to f2fs' block
size.

Testcase: (backend device: zram)
- touch file
- pin & fallocate file to 8GB
- mkswap file
- swapon file

Before:
real	0m2.999s
user	0m0.000s
sys	0m2.980s

After:
real	0m0.081s
user	0m0.000s
sys	0m0.064s
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>