sm8350-common: rootdir: fstab: Handle `avb=` fs_mgr flags correctly

Specifying avb=<vbmeta-partition> is only necessary for first entry of
partition groups that use same vbmeta partition. The following entry can have
<vbmeta-parititon> part omitted.

Test: None
Link: https://source.android.com/devices/architecture/kernel/mounting-partitions-early#fstab-entries


Signed-off-by: Albert I <kras@raphielgang.org>
Change-Id: I5a42fd3de72c400acfae56d6a1e1b85c22a79537