f2fs: Fix indefinite loop in f2fs_gc()

Policy - foreground GC, LFS mode and greedy GC mode.

Under this policy, f2fs_gc() loops forever to GC as it doesn't have
enough free segements to proceed and thus it keeps calling gc_more
for the same victim segment.  This can happen if the selected victim
segment could not be GC'd due to failed blkaddr validity check i.e.
is_alive() returns false for the blocks set in current validity map.

Fix this by not resetting the sbi->cur_victim_sec to NULL_SEGNO, when
the segment selected could not be GC'd. This helps to select another
segment for GC and thus helps to proceed forward with GC.

[Note]
This can happen due to is_alive as well as atomic_file which skipps
GC.

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>