f2fs: stop GC when the victim becomes fully valid

We must stop GC, once the segment becomes fully valid. Otherwise, it can
produce another dirty segments by moving valid blocks in the segment partially.

Ramon hit no free segment panic sometimes and saw this case happens when
validating reliable file pinning feature.

Signed-off-by: Ramon Pantin <pantin@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>