allow HW_USERMEM64 in sysctl pledge

[Jonathan Gray]

chromium loads vulkan when going to chrome://gpu
on Intel Mesa uses HW_USERMEM64

chrome(44801): pledge sysctl 2: 6 20
chrome[44801]: pledge "", syscall 202

Index: sys/kern/kern_pledge.c
===================================================================
RCS file: /cvs/src/sys/kern/kern_pledge.c,v
retrieving revision 1.281
diff -u -p -r1.281 kern_pledge.c
--- sys/kern/kern_pledge.c      25 Mar 2022 17:40:59 -0000      1.281
+++ sys/kern/kern_pledge.c      21 Jun 2022 06:23:09 -0000
@@ -977,6 +977,7 @@ pledge_sysctl(struct proc *p, int miblen
                        case HW_PHYSMEM64:      /* hw.physmem */
                        case HW_NCPU:           /* hw.ncpu */
                        case HW_NCPUONLINE:     /* hw.ncpuonline */
+                       case HW_USERMEM64:      /* hw.usermem */
                                return (0);
                        }
                        break;