sysfs, kernfs: move internal decls to fs/kernfs/kernfs-internal.h

Move data structure, constant and basic accessor declarations from
fs/sysfs/sysfs.h to fs/kernfs/kernfs-internal.h.  The two files
currently include each other.  Once kernfs / sysfs separation is
complete, the cross inclusions will be removed.  Inclusion protectors
are added to fs/sysfs/sysfs.h to allow cross-inclusion.

This patch doesn't introduce any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>