f2fs: fix to check readonly condition correctly

In the Linux kernel, the following vulnerability has been resolved:

f2fs: fix to check readonly condition correctly

With below case, it can mount multi-device image w/ rw option, however
one of secondary device is set as ro, later update will cause panic, so
let's introduce f2fs_dev_is_readonly(), and check multi-devices rw status
in f2fs_remount() w/ it in order to avoid such inconsistent mount status.

mkfs.f2fs -c /dev/zram1 /dev/zram0 -f
blockdev --setro /dev/zram1
mount -t f2fs dev/zram0 /mnt/f2fs
mount: /mnt/f2fs: WARNING: source write-protected, mounted read-only.
mount -t f2fs -o remount,rw mnt/f2fs
dd if=/dev/zero of=/mnt/f2fs/file bs=1M count=8192

kernel BUG at fs/f2fs/inline.c:258!
RIP: 0010:f2fs_write_inline_data+0x23e/0x2d0 [f2fs]
Call Trace:
f2fs_write_single_data_page+0x26b/0x9f0 [f2fs]
f2fs_write_cache_pages+0x389/0xa60 [f2fs]
__f2fs_write_data_pages+0x26b/0x2d0 [f2fs]
f2fs_write_data_pages+0x2e/0x40 [f2fs]
do_writepages+0xd3/0x1b0
__writeback_single_inode+0x5b/0x420
writeback_sb_inodes+0x236/0x5a0
__writeback_inodes_wb+0x56/0xf0
wb_writeback+0x2a3/0x490
wb_do_writeback+0x2b2/0x330
wb_workfn+0x6a/0x260
process_one_work+0x270/0x5e0
worker_thread+0x52/0x3e0
kthread+0xf4/0x120
ret_from_fork+0x29/0x50