fs/btrfs: Make extent item iteration to handle gaps

fs/btrfs: Make extent item iteration to handle gaps

The GRUB btrfs implementation can't handle two very basic btrfs
file layouts:

1. Mixed inline/regualr extents
   # mkfs.btrfs -f test.img
   # mount test.img /mnt/btrfs
   # xfs_io -f -c "pwrite 0 1k" -c "sync" -c "falloc 0 4k" \
	       -c "pwrite 4k 4k" /mnt/btrfs/file
   # umount /mnt/btrfs
   # ./grub-fstest ./grub-fstest --debug=btrfs ~/test.img hex "/file"

   Such mixed inline/regular extents case is not recommended layout,
   but all existing tools and kernel can handle it without problem.

2. NO_HOLES feature
   # mkfs.btrfs -f test.img -O no_holes
   # mount test.img /mnt/btrfs
   # xfs_io -f -c "pwrite 0 4k" -c "pwrite 8k 4k" /mnt/btrfs/file
   # umount /mnt/btrfs
   # ./grub-fstest ./grub-fstest --debug=btrfs ~/test.img hex "/file"

   NO_HOLES feature is going to be the default mkfs feature in the incoming
   v5.15 release, and kernel has support for it since v4.0.

The way GRUB btrfs code iterates through file extents relies on no gap
between extents.

If any gap is hit, then GRUB btrfs will error out, without any proper
reason to help debug the bug.

This is a bad assumption, since a long long time ago btrfs has a new
feature called NO_HOLES to allow btrfs to skip the padding hole extent
to reduce metadata usage.

The NO_HOLES feature is already stable since kernel v4.0 and is going to
be the default mkfs feature in the incoming v5.15 btrfs-progs release.

When there is a extent gap, instead of error out, just try next item.

This is still not ideal, as kernel/progs/U-boot all do the iteration
item by item, not relying on the file offset continuity.

But it will be way more time consuming to correct the whole behavior than
starting from scratch to build a proper designed btrfs module for GRUB.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>