By profiling f2fs compression works, I've found vmap() callings have
unexpected hikes in the execution time in our test environment and
those are bottlenecks of f2fs decompression path. Changing these with
vm_map_ram(), we can enhance f2fs decompression speed pretty much.

[Verification]
Android Pixel 3(ARM64, 6GB RAM, 128GB UFS)
Turned on only 0-3 little cores(at 1.785GHz)

dd if=/dev/zero of=dummy bs=1m count=1000
echo 3 > /proc/sys/vm/drop_caches
dd if=dummy of=/dev/zero bs=512k

- w/o compression -
1048576000 bytes (0.9 G) copied, 2.082554 s, 480 M/s
1048576000 bytes (0.9 G) copied, 2.081634 s, 480 M/s
1048576000 bytes (0.9 G) copied, 2.090861 s, 478 M/s

- before patch -
1048576000 bytes (0.9 G) copied, 7.407527 s, 135 M/s
1048576000 bytes (0.9 G) copied, 7.283734 s, 137 M/s
1048576000 bytes (0.9 G) copied, 7.291508 s, 137 M/s

- after patch -
1048576000 bytes (0.9 G) copied, 1.998959 s, 500 M/s
1048576000 bytes (0.9 G) copied, 1.987554 s, 503 M/s
1048576000 bytes (0.9 G) copied, 1.986380 s, 503 M/s
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>