[kernel] Allow passing 0 to vma_resize to query the current size

Passing a size of 0 in to vma_resize will now not attempt to alter the VMA size, but will
still put the size into the passed-in pointer. Using this allows querying the size of a
VMA without changing it.

definitions/objects/vma.def

@@ -32,6 +32,6 @@ object vma : object {
     }
 
     method resize [cap:resize] {
-        param size size [inout]
+        param size size [inout]  # New size for the VMA, or 0 to query the current size without changing
     }
 }

src/kernel/objects/vm_area.h

@@ -22,8 +22,8 @@ enum class vm_flags : uint32_t
 #define VM_FLAG(name, v) name = v,
 #include <j6/tables/vm_flags.inc>
 #undef VM_FLAG
-    driver_mask     = 0x000fffff, ///< flags allowed via syscall for drivers
+    driver_mask     = 0x00ff'ffff, ///< flags allowed via syscall for drivers
-    user_mask       = 0x0000ffff, ///< flags allowed via syscall for non-drivers
+    user_mask       = 0x000f'ffff, ///< flags allowed via syscall for non-drivers
 };
 is_bitfield(vm_flags);
 

src/kernel/syscalls/vm_area.cpp

@@ -56,7 +56,11 @@ vma_unmap(vm_area *self, process *proc)
 j6_status_t
 vma_resize(vm_area *self, size_t *size)
 {
+    if (!*size) {
+        *size = self->size();
+    } else {
         *size = self->resize(*size);
+    }
     return j6_status_ok;
 }