f2fs: fix to avoid running out of free segments

If checkpoint is disabled, GC can not reclaim any segments, we need
to detect such condition and bail out from fallocate() of a pinfile,
rather than letting allocator running out of free segment, which may
cause f2fs to be shutdown.

reproducer:
mkfs.f2fs -f /dev/vda 16777216
mount -o checkpoint=disable:10% /dev/vda /mnt/f2fs
for ((i=0;i<4096;i++)) do { dd if=/dev/zero of=/mnt/f2fs/$i bs=1M count=1; } done
sync
for ((i=0;i<4096;i+=2)) do { rm /mnt/f2fs/$i; } done
sync
touch /mnt/f2fs/pinfile
f2fs_io pinfile set /mnt/f2fs/pinfile
f2fs_io fallocate 0 0 4201644032 /mnt/f2fs/pinfile

cat /sys/kernel/debug/f2fs/status
output:
  - Free: 0 (0)

Fixes: f5a53edc ("f2fs: support aligned pinned file")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>