f2fs: use f2fs_hw_is_readonly() instead of bdev_read_only()

f2fs has supported multi-device feature, to check devices' rw status,
it should use f2fs_hw_is_readonly() rather than bdev_read_only(), fix
it.

Meanwhile, it removes f2fs_hw_is_readonly() check condition in:
- f2fs_write_checkpoint()
- f2fs_convert_inline_inode()
As it has checked f2fs_readonly() condition, and if f2fs' devices
were readonly, f2fs_readonly() must be true.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>