fs/Makefile: Always inspect exofs/

fs/Makefile: Always inspect exofs/

fs/exofs directory has multiple targets now, of which the
ore.ko will be needed by the pnfs-objects-layout-driver
(fs/nfs/objlayout).

As suggested by: Michal Marek <mmarek@suse.cz>  convert
inclusion of exofs/ from obj-$(CONFIG_EXOFS_FS) => obj-$(y).
So ORE can be selected also from fs/nfs/Kconfig

CC: Michal Marek <mmarek@suse.cz>
CC: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Boaz Harrosh <bharrosh@panasas.com>