Catch up with VM map reservations.

As of CTSRD-CHERI/cheribsd@d25a09e
we don't need to pad mmap requests, the kernel does it for us.

cheri-c-programming.tex

@@ -1525,10 +1525,11 @@ \subsection{POSIX API changes}
 
 \item[\cfunc{mmap} bounds] In CheriABI, the \cfunc{mmap} system
    call returns a bounded capability to the allocated address space.
-   The requested mapping must be rounded up in length to ensure that the
-   returned capability does not overlap with unallocated (or otherwise
-   allocated) regions.
-   This currently requires code changes in consumers of \cfunc{mmap}.
+   To ensure the capability does not overlap other allocations,
+   lengths that would otherwise be unrepresentable are rounded up
+   and padded with a new type of guard pages.
+   These guard pages fault on access and may not be mapped over.
+   They are unmapped when the rest of the mapping is unmapped.
 
 \item[\cfunc{mmap} permissions] The permissions of the capability
    returned by \cfunc{mmap} are determined by a combination of the