i-t: fix root=zfs:AUTO

IFS= would break loops in import_pool(), which would fault
any automatic import

Additionally $ZFS_BOOTFS from cmdline would interfere with find_rootfs()

If many pools were present, same thing could happen across multiple
find_rootfs() runs, so bail out early and clean up in error path

Suggested-by: @nachtgeist
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11278
Closes #11838