BACKPORT: f2fs: don't set GC_FAILURE_PIN for background GC

So that it can reduce the possibility that file be unpinned forcely by
foreground GC due to .i_gc_failures[GC_FAILURE_PIN] exceeds threshold.

Bug: 278486610
Bug: 279103780
Signed-off-by: Chao Yu <chao.yu@oppo.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Signed-off-by: Chao Yu <chao@kernel.org>
(cherry picked from commit 6c99804c)
Change-Id: Ic864b84870d689383f47ddb5e19b260e005974e0