um: Remove double zero check

free_pages() performs a parameter null check inside
therefore remove double zero check here.

Signed-off-by: Shaojie Dong <quic_shaojied@quicinc.com>
Link: https://patch.msgid.link/20241025-upstream_branch-v5-1-b8998beb2c64@quicinc.com


Signed-off-by: Johannes Berg <johannes.berg@intel.com>