Move kern_extattr_* prototypes to <sys/syscallsubr.h>

All of the kern_* prototypes belong in this header. While here, sort
the prototypes by function name.

Reviewed by: dchagin
Fixes: 6453d4240f6b vfs: Export exattr methods to reuse by Linuxulator
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D41766

(cherry picked from commit 3555be0124a4f105c72d932f00071f332691e8cf)