BACKPORT: f2fs: enforce the immutable flag on open files

commit e0fcd01510ad upstream.

This patch ports commit 02b016ca ("ext4: enforce the immutable
flag on open files") to f2fs.

According to the chattr man page, "a file with the 'i' attribute
cannot be modified..."  Historically, this was only enforced when the
file was opened, per the rest of the description, "... and the file
can not be opened in write mode".

There is general agreement that we should standardize all file systems
to prevent modifications even for files that were opened at the time
the immutable flag is set.  Eventually, a change to enforce this at
the VFS layer should be landing in mainline.

Bug: 237904436
Cc: stable@kernel.org
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Change-Id: I9c74498ed3174309ff14158c8c3ad6dee6e1c8ca