filemap_populate and shmem_populate must install even a linear file_pte,
in case there was a nonlinear page or file_pte already installed there:
could only happen if already VM_NONLINEAR, but no need to check that.

Acked by Ingo and Hugh.
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>