btrfs-progs: check: Export check global variables to check/common.h

btrfs-progs: check: Export check global variables to check/common.h

There are a dozen of variables which are used as "check global"
variables, like @total_csum_bytes or @no_holes.

These variables are used freely across the check code, however since
we're splitting check code, they need to be exported so they can be used
in other files.

This patch just export them and add declarations for them in
check/common.h.

Reviewed-by: Su Yue <suy.fnst@cn.fujitsu.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>