f2fs: avoid unnecessary search while finding victim in gc

variable nsearched in get_victim_by_default() indicates the number of
dirty segments we already checked. There are 2 problems about the way
it updates:
1. When p.ofs_unit is greater than 1, the victim we find consists
   of multiple segments, possibly more than 1 dirty segment.
   But nsearched always increases by 1.
2. If segments have been found but not been chosen, nsearched won't
   increase. So even we have checked all dirty segments, nsearched
   may still less than p.max_search.
All these problems could cause unnecessary search after all dirty
segments have already been checked.

Signed-off-by: Fan li <fanofcode.li@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>