btrfs: convert incompat and compat flag test helpers to macros

These helpers use functions not defined in fs.h, they're simply
accessors of the super block in fs_info, convert them to macros so
that we don't have a weird dependency between fs.h and accessors.h.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>