f2fs: fix to set flush_merge opt and show noflush_merge

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.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>