x86/mm: account for the offset when performing subpage r/o MMIO access

x86/mm: account for the offset when performing subpage r/o MMIO access

The current logic in subpage_mmio_write_emulate() doesn't take into account
the page offset, and always performs the writes at offset 0 (start of the
page).

Fix this by accounting for the offset before performing the write.

Fixes: 8847d6e23f97 ('x86/mm: add API for marking only part of a MMIO page read only')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>