snapuserd: use fadvise to invalidate pages for merge threads

Performance numbers measured on pixel watch:

Monitoring vmstat output during the window of snapshot merge for 280 seconds.

With patch:

pgsteal_kswapd  342144
pgsteal_file  386977

Without patch (baseline):

pgsteal_kswapd 1878334
pgsteal_file 1854342

Clearly kswapd activity is reduced. Additionally no direct reclaim observed from vmstat output.

Bug: 425765067
Test: Incremental OTA on Pixel watch
Change-Id: I4455973fdc81ab2b38671e34b60c1cf3ce6eff19
Signed-off-by: Akilesh Kailash <akailash@google.com>