mm: add RCU annotation to pte_offset_map(_lock)

RCU lock is taken by ___pte_offset_map() unless it returns NULL.  Add this
information to its inline callers to avoid sparse warning about context
imbalance in pte_unmap().

Link: https://lkml.kernel.org/r/20241210000604.700710-1-oss@malat.biz


Signed-off-by: Petr Malat <oss@malat.biz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>