procfs: call default get_unmapped_area on MMU-present architectures

procfs: call default get_unmapped_area on MMU-present architectures

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.

Reported-by: Michael Holzheu <[email protected]>
Signed-off-by: HATAYAMA Daisuke <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: David S. Miller <[email protected]>
Tested-by: Michael Holzheu <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>