runtime: add missing unlock in sysReserveAlignedSbrk

sysReserveAlignedSbrk locks memlock at entry, but it is not
unlocked at one of the return path. Add the missing unlock.

Fixes #74339.

Change-Id: Ib641bc348aca41494ec410e2c4eb9857f3362484
Reviewed-on: https://go-review.googlesource.com/c/go/+/683295
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/runtime/mem_sbrk.go

@@ -231,6 +231,7 @@ func sysReserveAlignedSbrk(size, align uintptr) (unsafe.Pointer, uintptr) {
 			memFree(unsafe.Pointer(end), endLen)
 		}
 		memCheck()
+		unlock(&memlock)
 		return unsafe.Pointer(pAligned), size
 	}