LU-12784 tests: fix large_xattr_enabled() for ZFS

LU-12784 tests: fix large_xattr_enabled() for ZFS

Fix large_xattr_enabled() check for ZFS filesystems, since bash
functions return "0" for true.  Otherwise, all ZFS tests that
check large_xattr_enabled() will be skipped.

Fixes: 84097792f56c ("LU-12784 llite: limit max xattr size by kernel value")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Ie566244c6b1f46b947a96331e7623b9b863ebbe5
Reviewed-on: https://review.whamcloud.com/45264
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Reviewed-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>