f2fs: expose f2fs_write_cache_pages

If there are gced dirty pages and normal dirty pages in the mapping
of one inode, we might writeback them alternately with discontinuous
block address, resulting in low performance.

This patch introduces f2fs_write_cache_pages with codes copied from
write_cache_pages in mm/page-writeback.c.

In this function, we refactor flow with two steps:
1) writeback all cold type pages.
2) writeback all non-cold type pages.

By using this method, f2fs will writeback dirty pages with the same
temperature in bunch mode, it makes writeouted block being with
more continuous address, so they can be merged as much as possible
in f2fs bio cache, and also it will reduce the chance of submiting
small IO from block layer.

Test environment: 8g nokia sd card (very old sd card, but it shows
better effect when testing with this patch, and with a 32g kingston
sd card, I didn't see much more improvement).

Test step:
1. touch testfile;
2. truncate -s 512K testfile;
3. write all pages with odd index;
4. trigger gc by ioctl;
5. write all pages with even index;
6. time fsync testfile.

before:
real	0m0.402s
user	0m0.000s
sys	0m0.000s

after:
real	0m0.143s
user	0m0.004s
sys	0m0.004s

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>