f2fs: compress: fix to avoid fragment w/ OPU during f2fs_ioc_compress_file()

If file has both cold and compress flag, during f2fs_ioc_compress_file(),
f2fs will trigger IPU for non-compress cluster and OPU for compress
cluster, so that, data of the file may be fragmented.

Fix it by always triggering OPU for IOs from user mode compression.

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