fs/btrfs: Use full btrfs bootloader area

fs/btrfs: Use full btrfs bootloader area

Up to now GRUB can only embed to the first 64 KiB before primary
superblock of btrfs, effectively limiting the GRUB core size. That
could consequently pose restrictions to feature enablement like
advanced zstd compression.

This patch attempts to utilize full unused area reserved by btrfs for
the bootloader outlined in the document [1]:

  The first 1MiB on each device is unused with the exception of primary
  superblock that is on the offset 64KiB and spans 4KiB.

Apart from that, adjacent sectors to superblock and first block group
are not used for embedding in case of overflow and logged access to
adjacent sectors could be useful for tracing it up.

This patch has been tested to provide out of the box support for btrfs
zstd compression with which GRUB has been installed to the partition.

[1] https://btrfs.wiki.kernel.org/index.php/Manpage/btrfs(5)#BOOTLOADER_SUPPORT

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>