f2fs: fix f2fs_show_options to show nogc_merge mount option

Commit 5911d2d1 ("f2fs: introduce gc_merge mount option") forgot
to show nogc_merge option, let's fix it.

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