rdonly: Added missing isrdonly flag overrides when LFS3_RDONLY

I did override lfs3_o_isrdonly, but missed lfs3_m_isrdonly and
lfs3_t_isrdonly.

These aren't strictly necessary (asserts force rdonly flags to be set
correctly), but can save code by trimming unreachable code paths.

That being said, currently no observable code savings:

                  code          stack          ctx
  rdonly before: 10664            840          524
  rdonly after:  10664 (+0.0%)    840 (+0.0%)  524 (+0.0%)

But I noticed while toying around with a different way of pruning
LFS3_TSTATE_OMDIRS/OBTREE and wanted to make sure other code savings
weren't dragged in.

lfs3.c

@@ -6926,7 +6926,12 @@ static inline bool lfs3_a_islazy(uint32_t flags) {
 
 // traversal flags
 static inline bool lfs3_t_isrdonly(uint32_t flags) {
+    (void)flags;
+    #ifndef LFS3_RDONLY
     return flags & LFS3_T_RDONLY;
+    #else
+    return true;
+    #endif
 }
 
 static inline bool lfs3_t_ismtreeonly(uint32_t flags) {
@@ -7018,7 +7023,12 @@ static inline uint32_t lfs3_t_swapdirty(uint32_t flags) {
 
 // mount flags
 static inline bool lfs3_m_isrdonly(uint32_t flags) {
+    (void)flags;
+    #ifndef LFS3_RDONLY
     return flags & LFS3_M_RDONLY;
+    #else
+    return true;
+    #endif
 }
 
 #ifdef LFS3_REVDBG