Patch by Christoph Hellwig.

This patch is cleanup of mmap.c with the side-effect of making the lock
order of mapping->i_shared_lock and mm->page_table_lock consistant.

__remove_shared_vm_struct is merged into its only caller,
remove_shared_vm_struct, which now also does the job of
{lock,unlock}_vma_mappings itself, to avoid duplicate checks.

Similarly, vma_link is updated to take and release i_shared_lock
itself, fixing the lock order.  split_vma is updated to use
insert_vm_struct instead of using __insert_vm_struct and taking
page_table_lock and i_shared_lock itself (wrongly).  __insert_vm_struct
is removed as it is unused now.