Аdded a return value check for the function
mmu_notifier_register().

Return value of a function 'mmu_notifier_register'
called at iommu_v2.c:642 is not checked,
 but it is usually checked for this function

Found by Linux Verification Center (linuxtesting.org) with SVACE.
Signed-off-by: Denis Arefev <arefev@swemel.ru>
Link: https://lore.kernel.org/r/20221118104252.122809-1-arefev@swemel.ru
[joro: Fix commit message ]
Signed-off-by: Joerg Roedel <jroedel@suse.de>