btrfs: tree-checker: add checker for items in remap tree

btrfs: tree-checker: add checker for items in remap tree

Add write-time checking of items in the remap tree, to catch errors
before they are written to disk.

We're checking:

* That remap items, remap backrefs, and identity remaps aren't written
  unless the REMAP_TREE incompat flag is set
* That identity remaps have a size of 0
* That remap items and remap backrefs have a size of sizeof(struct
  btrfs_remap_item)
* That the objectid for these items is aligned to the sector size
* That the offset for these items (i.e. the size of the remapping) isn't
  0 and is aligned to the sector size
* That objectid + offset doesn't overflow

Signed-off-by: Mark Harmstone <mark@harmstone.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>