x86/pv: Minor improvements to guest_get_eff_{,kern}_l1e()

x86/pv: Minor improvements to guest_get_eff_{,kern}_l1e()

 * These functions work in terms of linear addresses, not virtual addresses.
   Update the comments and parameter names.
 * Drop unnecessary inlines.
 * Drop vcpu parameter from guest_get_eff_kern_l1e().  Its sole caller passes
   current, and its callee strictly operates on current.
 * Switch guest_get_eff_kern_l1e()'s parameter from void * to l1_pgentry_t *.
   Both its caller and callee already use the correct type already.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>