staging/lustre: lloop depends on BLOCK

Add a config option for llite/lloop in lustre driver, making it depends
on BLOCK to fix this better:
drivers/staging/lustre/lustre/fid/../include/linux/lustre_compat25.h:117:2:
error: implicit declaration of function ‘unregister_blkdev'

Also, remove the wrapper ll_unregister_blkdev which depends on BLOCK in
the header and just call unregister_blkdev in lloop.c based on Peng Tao's
comment. Drop the redundant dependency on STAGING for LUSTRE_FS, remove
some unnecessary jdb header files which depends on BLOCK btw.

Signed-off-by: Xiong Zhou <jencce.kernel@gmail.com>
Reviewed-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>