xen/trace: Fix trace metadata page count calculation (revert fbf96e6)

xen/trace: Fix trace metadata page count calculation (revert fbf96e6)

Changeset fbf96e6, "xentrace: correct formula to calculate
t_info_pages", broke the trace metadata page count calculation, by
mistaking t_info_first_offset as denominated in bytes, when in fact it
is denominated in words (uint32_t).

Effectively revert that change, and put a comment there to reduce the
chance that someone will make that mistake in the future.

Reviewed-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Tested-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Signed-off-by: George Dunlap <george.dunlap@citrix.com>