devices: fs: Replace RwLock with Mutex
devices: fs: Replace RwLock with Mutex

The RwLock implementation on linux has a tendency to starve writers.
Since the fs device workload has a roughly evenly distributed number of
reads and writes, using a Mutex should give us better performance.

BUG=b:150264042
TEST=vm.Blogbbench.virtiofs

Change-Id: I85ec4053bf03d19bb21b420c0aa506720e666708
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/2103604
Auto-Submit: Chirantan Ekbote <35787c0e27de5c48735e07f8324823b765e8bcbc@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>
Reviewed-by: Stephen Barber <7719587e4b5a49340661b214ec85602f3918d06f@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm