ANDROID: 16K: Avoid and document padding madvise lock warning

Usually to modify vm_flags we need to take exclusive mmap_lock but here
only have the lock in read mode, to avoid all DONTNEED/DONTNEED_LOCKED
calls needing the write lock.

A race to the flags update can only happen with another MADV_DONTNEED on
the same process and same range (VMA).

In practice, this specific scenario is not possible  because the action
that could cause it is usually performed at most once per VMA and only by the dynamic linker.

Forego protection for this case, to avoid penalties in the common cases.

Bug: 344634072
Change-Id: I54ac1f204e0445291f3df3872fbaa16b37722812
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>