Avoid null pointer dereference in dsl_fs_ss_limit_check()

Check for cr == NULL before dereferencing it in
dsl_enforce_ds_ss_limits() to lookup the zone/jail ID.

Reported-by: Coverity (CID 1210459)
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes #14103