procfs: also fix proc_reg_get_unmapped_area() for !MMU case

procfs: also fix proc_reg_get_unmapped_area() for !MMU case

Commit fad1a86e25e0 ("procfs: call default get_unmapped_area on
MMU-present architectures"), as its title says, took care of only the
MMU case, leaving the !MMU side still in the regressed state (returning
-EIO in all cases where pde->proc_fops->get_unmapped_area is NULL).

From the fad1a86e25e0 changelog:

 "Commit c4fe24485729 ("sparc: fix PCI device proc file mmap(2)") added
  proc_reg_get_unmapped_area in proc_reg_file_ops and
  proc_reg_file_ops_no_compat, by which now mmap always returns EIO if
  get_unmapped_area method is not defined for the target procfs file, which
  causes regression of mmap on /proc/vmcore.

  To address this issue, like get_unmapped_area(), call default
  current->mm->get_unmapped_area on MMU-present architectures if
  pde->proc_fops->get_unmapped_area, i.e.  the one in actual file operation
  in the procfs file, is not defined"

Signed-off-by: Jan Beulich <[email protected]>
Cc: HATAYAMA Daisuke <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: <[email protected]>	[3.12.x]
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>