f2fs: allow compression for mmap files in compress_mode=user

Since commit e3c54832 ("f2fs: let's allow compression for mmap files"),
it has been allowed to compress mmap files. However, in compress_mode=user,
it is not allowed yet. To keep the same concept in both compress_modes,
f2fs_ioc_(de)compress_file() should also allow it.

Let's remove checking mmap files in f2fs_ioc_(de)compress_file() so that
the compression for mmap files is also allowed in compress_mode=user.

Signed-off-by: Sungjong Seo <sj1557.seo@samsung.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>