sysctl: Refactor the binary sysctl handling to remove duplicate code

Read in the binary sysctl path once, instead of reread it
from user space each time the code needs to access a path
element.

The deprecated sysctl warning is moved to do_sysctl so
that the compat_sysctl entries syscalls will also warn.

The return of -ENOSYS when !CONFIG_SYSCTL_SYSCALL is moved
to binary_sysctl.  Always leaving a do_sysctl available
that handles !CONFIG_SYSCTL_SYSCALL and printing the
deprecated sysctl warning allows for a single defitition
of the sysctl syscall.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>