LU-14281 tests: fix sanity-sec test_56 for ZFS

LU-14281 tests: fix sanity-sec test_56 for ZFS

Skip sanity-sec test_56 in case of ZFS backend, as it does not
support FIEMAP.
Also simplify sanity-sec test_56, as all we care about is whether the
"encrypted" flag is set at all.

Fixes: 33322f3a2488 ("LU-14149 llite: fiemap set flags for encrypted files")
Test-Parameters: trivial
Test-Parameters: testlist=sanity-sec env=ONLY=56 clientdistro=el8.3 serverdistro=el8.3 fstype=ldiskfs
Test-Parameters: testlist=sanity-sec env=ONLY=56 clientdistro=el8.3 serverdistro=el8.3 fstype=zfs
Signed-off-by: Sebastien Buisson <sbuisson@ddn.com>
Change-Id: I69125d3e06acb89dbda421e573d18a2c22beac37
Reviewed-on: https://review.whamcloud.com/41130
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>