LU-1199 build: Cleanup ldiskfs kernel config defines

Make it more clear what the purpose of the "CONFIG_LDISKFS_*"
defines are with a comment, and separate them from options
that do not necessarily originate in the kernel source. All of
these options are needed for ldiskfs and some are needed by
osd-ldiskfs in the lustre file system. To handle these shared
options a extra autoconf header file is create containing only
the values of interest to osd-ldiskfs.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: I8e71b6cf471c4317b20fdce14d66f8b2883a226e
Reviewed-on: http://review.whamcloud.com/5675
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>