ghes_read_estatus() reads the record address, then the record's
header, then performs some sanity checks before reading the
records into the provided estatus buffer.

To provide this estatus buffer the caller must know the size of the
records in advance, or always provide a worst-case sized buffer as
happens today for the non-NMI notifications.

Add a function to peek at the record's header to find the size. This
will let the NMI path allocate the right amount of memory before reading
the records, instead of using the worst-case size, and having to copy
the records.

Split ghes_read_estatus() to create __ghes_peek_estatus() which
returns the address and size of the CPER records.
Signed-off-by: James Morse <james.morse@arm.com>

Changes since v7:
 * Grammar
 * concistent argument ordering

Changes since v6:
 * Additional buf_addr = 0 error handling
 * Moved checking out of peek-estatus
 * Reworded an error message so we can tell them apart
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>