[PATCH] fix remap of shared read only mappings

[PATCH] fix remap of shared read only mappings

When mmap MAP_SHARED is done on a file, it gets marked with VM_MAYSHARE
and, if it's read/write, VM_SHARED.  However, if it is remapped with
mremap(), the MAP_SHARED is only passed into the new mapping based on
VM_SHARED.  This means that remapped read only MAP_SHARED mappings lose
VM_MAYSHARE.  This is causing us a problem on parisc because we have to
align all shared mappings carefully to mitigate cache aliasing problems.

The fix is to key passing the MAP_SHARED flag back into the remapped are
off VM_MAYSHARE not VM_SHARED.