ANDROID: look up vma under RCU in linker_ctx()

madvise_dontneed_single_vma() calls linker_ctx() to detect whether the
madvise was initiated by the dynamic linker. This function requires
mmap_lock in order to lookup the vma, however with recent changes we
do not hold mmap_lock while executing MADV_DONTNEED. Lookup the vma
under RCU instead to avoid lockdep warning.

Bug: 425827225
Change-Id: Ie5e0243f359b96292d4f32ee3299050b871dc6c5
Signed-off-by: Suren Baghdasaryan <surenb@google.com>