xen/grant-table: Fix incorrect gnttab_dma_free_pages() pr_debug message

If a call to xenmem_reservation_increase() in gnttab_dma_free_pages()
fails it triggers a message "Failed to decrease reservation..." which
should be "Failed to increase reservation..."

Fixes: 9bdc7304 ('xen/grant-table: Allow allocating buffers suitable for DMA')
Reported-by: Ross Philipson <ross.philipson@oracle.com>
Signed-off-by: Liam Merwick <liam.merwick@oracle.com>
Reviewed-by: Mark Kanda <mark.kanda@oracle.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>