f2fs: avoid f2fs_map_blocks() for consecutive holes in readpages

f2fs: avoid f2fs_map_blocks() for consecutive holes in readpages

For consecutive large hole mapping across {d,id,did}nodes , we don't
need to call f2fs_map_blocks() to check one hole block per one time,
instead, we can use map.m_next_pgofs as a hint of next potential valid
block, so that we can skip calling f2fs_map_blocks the range of
[cur_pgofs + 1, .m_next_pgofs).

1) regular case

touch /mnt/f2fs/file
truncate -s $((1024*1024*1024)) /mnt/f2fs/file
time dd if=/mnt/f2fs/file of=/dev/null bs=1M count=1024

Before:
real    0m0.706s
user    0m0.000s
sys     0m0.706s

After:
real    0m0.620s
user    0m0.008s
sys     0m0.611s

2) large folio case

touch /mnt/f2fs/file
truncate -s $((1024*1024*1024)) /mnt/f2fs/file
f2fs_io setflags immutable /mnt/f2fs/file
sync
echo 3 > /proc/sys/vm/drop_caches
time dd if=/mnt/f2fs/file of=/dev/null bs=1M count=1024

Before:
real    0m0.438s
user    0m0.004s
sys     0m0.433s

After:
real    0m0.368s
user    0m0.004s
sys     0m0.364s

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