From: Hugh Dickins <hugh@veritas.com>

Why should struct address_space have separate i_mmap and i_mmap_shared
prio_trees (separating !VM_SHARED and VM_SHARED vmas)?  No good reason, the
same processing is usually needed on both.  Merge i_mmap_shared into i_mmap,
but keep i_mmap_writable count of VM_SHARED vmas (those capable of dirtying
the underlying file) for the mapping_writably_mapped test.

The VM_MAYSHARE test in the arm and parisc loops is not necessarily what they
will want to use in the end: it's provided as a harmless example of what might
be appropriate, but maintainers are likely to revise it later (that parisc
loop is currently being changed in the parisc tree anyway).

On the way, remove the now out-of-date comments on vm_area_struct size.