[ppc64] prevent infinite loop on icache sync

At moea64_sync_icache(), when the 'va' argument has page size
alignment, round_page() will return the same value as 'va'.
This would cause 'len' to be 0 and thus an infinite loop.

With this change, 'lim' will always point to the next page boundary.

This issue occurred especially during debugging sessions, when a breakpoint
was placed on an exact page-aligned offset, for instance.

Reviewed by: jhibbits
Differential Revision: https://reviews.freebsd.org/D19149