fs_mgr: add fs_mgr_do_mount_one() API

This is to be used in early mount case where we will have a fully
prepared fstab_rec. fs_mgr_do_mount() does a lot more checks and
spends time preparing verity / avb devices before it does the actual
mount.

b/33254008

Test: Boot sailfish

Change-Id: I4481b5af8d900c8b7e3355b7513c325d8f2ecff2
Signed-off-by: Sandeep Patil <sspatil@google.com>