md: namespace private helper names

The md private helper uuid_equal() collides with a generic helper
of the same name.

Rename the md private helper to md_uuid_equal() and do the same for
md_sb_equal().

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Shaohua Li <shli@kernel.org>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>