EFI partitioning scheme was reading the last reported sector of the
block device to look for the alternate GPT header, before it had
confirmed that it should.  This causes problems for devices with the
following problems:  a) those who misreport their size (typically
off-by-one), and b) those who fail when asked to read a block
outside their range.

This patch moves the test for the Protective Master Boot Record (PMBR)
ahead of the tests for the Primary and Alternate GPT headers.  If the
PMBR is not valid, the disk is assumed to not be a GPT disk.  This can
be overridden with the 'gpt' kernel command line option.  If the
Primary GPT header is not valid, the Alternate GPT header is not
probed automatically unless the 'gpt' kernel command line option is
passed.  If the both the PMBR and Primary GPT header are valid, then
the Alternate GPT header at the end of the disk is probed.

Also re-enables CONFIG_EFI_PARTITION for all architectures.
Signed-off-by: Matt Domsch <Matt_Domsch@dell.com>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>