UML - use correct register file size everywhere

UML - use correct register file size everywhere

This patch uses MAX_REG_NR consistently to refer to the register file
size.  FRAME_SIZE isn't sufficient because on x86_64, it is smaller
than the ptrace register file size.  MAX_REG_NR was introduced as a
consistent way to get the number of registers, but wasn't used
everywhere it should be.

When this causes a problem, it makes PTRACE_SETREGS fail on x86_64
because of a corrupted segment register value in the known-good
register file. The patch also adds a register dump at that point in
case there are any future problems here.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>