fs/btrfs: Add environment block to reserved header area

This patch reserves space for the GRUB environment block inside the
Btrfs header. The block is placed at an offset of GRUB_ENV_BTRFS_OFFSET,
256 KiB from the start of the device, and occupies one sector. To
protect the space, overflow guard sectors are placed before and after
the reserved block.

The Btrfs header already defines regions for bootloader use. By adding
this entry, GRUB gains a fixed and safe location to store the environment
block without conflicting with other structures in the header.

Add Btrfs and its reserved area information to the fs_envblk_spec table.
With the groundworks done in previous patches, the function is now
complete and working in grub-editenv.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Neal Gompa <ngompa13@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

include/grub/fs.h

@@ -132,4 +132,6 @@ grub_fs_unregister (grub_fs_t fs)
 
 grub_fs_t EXPORT_FUNC(grub_fs_probe) (grub_device_t device);
 
+#define GRUB_ENV_BTRFS_OFFSET (256 * 1024)
+
 #endif /* ! GRUB_FS_HEADER */