Allow use of pool GUID as root pool

It's helpful if there are pools with same names,
but you need to use only one of them.

Main case is twin servers, meanwhile some software
requires the same name of pools (e.g. Proxmox).

Reviewed-by: Kash Pande <kash@tripleback.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Igor ‘guardian’ Lidin of Moscow, Russia
Closes #8052

contrib/initramfs/scripts/zfs.in

@@ -193,7 +193,7 @@ import_pool()
 
 	# Verify that the pool isn't already imported
 	# Make as sure as we can to not require '-f' to import.
-	"${ZPOOL}" status "$pool" > /dev/null 2>&1 && return 0
+	"${ZPOOL}" get name,guid -o value -H 2>/dev/null | grep -Fxq "$pool" && return 0
 
 	# For backwards compatibility, make sure that ZPOOL_IMPORT_PATH is set
 	# to something we can use later with the real import(s). We want to
@@ -772,6 +772,7 @@ mountroot()
 	#	root=zfs:<pool>/<dataset>	(uses this for rpool - first part, without 'zfs:')
 	#
 	# Option <dataset> could also be <snapshot>
+	# Option <pool> could also be <guid>
 
 	# ------------
 	# Support force option
@@ -889,6 +890,14 @@ mountroot()
 		/bin/sh
 	fi
 
+	# In case the pool was specified as guid, resolve guid to name
+	pool="$("${ZPOOL}" get name,guid -o name,value -H | \
+	    awk -v pool="${ZFS_RPOOL}" '$2 == pool { print $1 }')"
+	if [ -n "$pool" ]; then
+		ZFS_BOOTFS="${pool}/${ZFS_BOOTFS#*/}"
+		ZFS_RPOOL="${pool}"
+	fi
+
 	# Set elevator=noop on the root pool's vdevs' disks.  ZFS already
 	# does this for wholedisk vdevs (for all pools), so this is only
 	# important for partitions.