f2fs: fix to set flush_merge opt and show noflush_merge

[ Upstream commit 967eaad1 ]

Some minor modifications to flush_merge and related parameters:

  1.The FLUSH_MERGE opt is set by default only in non-ro mode.
  2.When ro and merge are set at the same time, an error is reported.
  3.Display noflush_merge mount opt.

Suggested-by: Chao Yu <chao@kernel.org>
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: 458c15df ("f2fs: don't reset unchangable mount option in f2fs_remount()")
Signed-off-by: Sasha Levin <sashal@kernel.org>