xfstests: make fs for 274 larger

xfstests: make fs for 274 larger

Btrfs will default to mixed block groups for 1 gigabyte file systems and
smaller, which means data and metadata share the same area.  This makes
generic/274 fail for us because we cannot reserve enough metadata space to do
our writes.  Bumping the scratch fs up to 2 gigabytes allows us to do our normal
metadata/data separation and allows us to pass this test.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>