Fix failure on mounting system_ext partition

There can be more than one fstab entry of system partition.
For example, the filesystem of one entry is ext4 and another is erofs.

system /system ext4 ro wait,slotselect,avb=vbmeta_system,logical,first_stage_mount,avb_keys=/avb/q-gsi.avbpubkey:/avb/r-gsi.avbpubkey:/avb/s-gsi.avbpubkey
system /system erofs ro wait,slotselect,avb=vbmeta_system,logical,first_stage_mount,avb_keys=/avb/q-gsi.avbpubkey:/avb/r-gsi.avbpubkey:/avb/s-gsi.avbpubkey

If both filesystems of system and system_ext paritions are erofs,
only the ext4 fstab entry will be returned by GetEntryForMountPoint,
and system_ext cann't be mounted.

So we need to return both of the fstab entries and try all of them.

Signed-off-by: Lianjun Huang <huanglianjun@xiaomi.com>
Change-Id: I407553d48b1749cd0554f057a6bfd38daa96fdcb
Signed-off-by: Lianjun Huang <huanglianjun@xiaomi.corp-partner.google.com>