fstab: add slotselect fsmgr arg

This causes the correct slot suffix to be appended to the partition
name. On non-A/B devices (like the PocoPhone F1) this should be ignored
because the slot_suffix arg doesn't exist.
diff --git a/sdm845/fstab.ramdisk b/sdm845/fstab.ramdisk
index 17bf875..4d972e7 100644
--- a/sdm845/fstab.ramdisk
+++ b/sdm845/fstab.ramdisk
@@ -1,4 +1,4 @@
-/dev/block/platform/soc@0/1d84000.ufshc/by-name/system_a /system ext4 noatime,ro,errors=panic wait,first_stage_mount
-/dev/block/platform/soc@0/1d84000.ufshc/by-name/vendor_a /vendor ext4 noatime,ro,errors=panic wait,first_stage_mount
+/dev/block/platform/soc@0/1d84000.ufshc/by-name/system /system ext4 noatime,ro,errors=panic wait,first_stage_mount,slotselect
+/dev/block/platform/soc@0/1d84000.ufshc/by-name/vendor /vendor ext4 noatime,ro,errors=panic wait,first_stage_mount,slotselect
 /dev/block/platform/soc@0/1d84000.ufshc/by-name/userdata /data		ext4	discard,noatime,noauto_da_alloc,data=ordered,user_xattr,barrier=1	wait,formattable,quota
 /devices/platform/soc@0/8804000.sdhci/mmc_host/mmc*	auto	auto	defaults	voldmanaged=sdcard1:auto