Linux 3.10 compat: Do not rely on struct proc_dir_entry definition

torvalds/linux#59d8053f moved the definition of struct proc_dir_entry
from include/linux/proc_fs.h to fs/proc/internal.h. The SPL relied on
that to map Solaris' kstat to entries in /proc/spl/kstat. Brian
Belendorf suggested reworking "the __kstat_install/__kstat_delete
handlers to only pass around the opaque handle", which is what we do.

Signed-off-by: Richard Yao <ryao@gentoo.org>