f2fs: introduce gc_mode sysfs node

Revert "f2fs: make gc_urgent and gc_segment_mode sysfs node readable".

Add a gc_mode sysfs node to show the current gc_mode as a string.

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