f2fs: support errors=remount-ro|continue|panic mountoption

This patch supports errors=remount-ro|continue|panic mount option
for f2fs.

f2fs behaves as below in three different modes:
mode			continue	remount-ro	panic
access ops		normal		noraml		N/A
syscall errors		-EIO		-EROFS		N/A
mount option		rw		ro		N/A
pending dir write	keep		keep		N/A
pending non-dir write	drop		keep		N/A
pending node write	drop		keep		N/A
pending meta write	keep		keep		N/A

By default it uses "continue" mode.

[Yangtao helps to clean up function's name]
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>