nommu: fix SYSV SHM for NOMMU

Commit c4caa778 ("file
->get_unmapped_area() shouldn't duplicate work of get_unmapped_area()")
broke SYSV SHM for NOMMU by taking away the pointer to
shm_get_unmapped_area() from shm_file_operations.

Put it back conditionally on CONFIG_MMU=n.

file->f_ops->get_unmapped_area() is used to find out the base address for a
mapping of a mappable chardev device or mappable memory-based file (such as a
ramfs file).  It needs to be called prior to file->f_ops->mmap() being called.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Cc: Greg Ungerer <gerg@snapgear.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>