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

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Linux kernel 安全漏洞

Linux kernel是美国Linux基金会的开源操作系统Linux所使用的内核。 Linux kernel存在安全漏洞，该漏洞源于f2fs未正确检查多设备只读条件，可能导致空指针取消引用。

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