f2fs: add helper to check compression level

commit c571fbb5 upstream.

This patch adds a helper function to check if compression level is
valid.

Meanwhile, this patch fixes a reported issue [1]:

The issue is easily reproducible by:

1. dd if=/dev/zero of=test.img count=100 bs=1M
2. mkfs.f2fs -f -O compression,extra_attr ./test.img
3. mount -t f2fs -o compress_algorithm=zstd:6,compress_chksum,atgc,gc_merge,lazytime ./test.img /mnt

resulting in

[   60.789982] F2FS-fs (loop0): invalid zstd compress level: 6

A bugzilla report has been submitted in
https://bugzilla.kernel.org/show_bug.cgi?id=218471

[1] https://lore.kernel.org/lkml/ZcWDOjKEnPDxZ0Or@google.com/T/



The root cause is commit 00e120b5 ("f2fs: assign default compression
level") tries to check low boundary of compress level w/ zstd_min_clevel(),
however, since commit e0c1b49f ("lib: zstd: Upgrade to latest upstream
zstd version 1.4.10"), zstd supports negative compress level, it cast type
for negative value returned from zstd_min_clevel() to unsigned int in below
check condition, result in repored issue.

	if (level < zstd_min_clevel() || ...

This patch fixes this issue by casting type for level to int before
comparison.

Fixes: 00e120b5 ("f2fs: assign default compression level")
Signed-off-by: Sheng Yong <shengyong@oppo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>