The mount helper mount.zfs MUST be in /sbin (not '$sbindir').

Commit 60e9f69 added the --with-mounthelperdir option for Gentoo
and in the process accidentally modified the default installation
location.  For security reasons mount(8) expects it to only be
installed under /sbin.

Signed-off-by: Turbo Fredriksson <turbo@bayour.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3426

config/mount-helper.m4

@@ -2,7 +2,7 @@ AC_DEFUN([ZFS_AC_CONFIG_USER_MOUNT_HELPER], [
 	AC_ARG_WITH(mounthelperdir,
 		AC_HELP_STRING([--with-mounthelperdir=DIR],
 		[install mount.zfs in dir [[/sbin]]]),
-		mounthelperdir=$withval,mounthelperdir=$sbindir)
+		mounthelperdir=$withval,mounthelperdir=/sbin)
 
 	AC_SUBST(mounthelperdir)
 ])