xen/grant-table: refactor error cleanup in grow_gnttab_list()

The cleanup loop in grow_gnttab_list() is safe from the underflow of
the unsigned 'i' since nr_glist_frames is >= 1, but refactor it
anyway.

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>