f2fs: remove unused GC_FAILURE_PIN

After commit 3db1de0e ("f2fs: change the current atomic write way"),
we removed all GC_FAILURE_ATOMIC usage, let's change i_gc_failures[]
array to i_pin_failure for cleanup.

Meanwhile, let's define i_current_depth and i_gc_failures as union
variable due to they won't be valid at the same time.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>