configure: zfs: enable on Linux

ZFS-on-Linux implementation of ZFS starting with version 0.6.4
contains all the features we use. Additionally, as we support
'volmode' option handling that's not available on ZoL but is
available on FreeBSD, there is no need to block ZFS storage driver
on Linux anymore.

So un-mark zfs storage driver as FreeBSD-only.

configure.ac

@@ -2011,14 +2011,6 @@ if test "$with_storage_gluster" = "yes"; then
 fi
 AM_CONDITIONAL([WITH_STORAGE_GLUSTER], [test "$with_storage_gluster" = "yes"])
 
-if test "$with_storage_zfs" = "check"; then
-    with_storage_zfs=$with_freebsd
-fi
-
-if test "$with_storage_zfs" = "yes" && test "$with_freebsd" = "no"; then
-    AC_MSG_ERROR([The ZFS storage driver can be enabled on FreeBSD only.])
-fi
-
 if test "$with_storage_zfs" = "yes" ||
    test "$with_storage_zfs" = "check"; then
   AC_PATH_PROG([ZFS], [zfs], [], [$PATH:/sbin:/usr/sbin])