f2fs: clean up i_compress_flag and i_compress_level usage

.i_compress_level was introduced by commit 3fde13f8

 ("f2fs: compress:
support compress level"), but never be used.

This patch updates as below:
- load high 8-bits of on-disk .i_compress_flag to in-memory .i_compress_level
- load low 8-bits of on-disk .i_compress_flag to in-memory .i_compress_flag
- change type of in-memory .i_compress_flag from unsigned short to unsigned
char.

w/ above changes, we can avoid unneeded bit shift whenever during
.init_compress_ctx(), and shrink size of struct f2fs_inode_info.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>