xfs: use the common helper uuid_is_null()

Use the common helper uuid_is_null() and remove the xfs specific
helper uuid_is_nil().

The common helper does not check for the NULL pointer value as
xfs helper did, but xfs code never calls the helper with a pointer
that can be NULL.

Conform comments and warning strings to use the term 'null uuid'
instead of 'nil uuid', because this is the terminology used by
lib/uuid.c and its users. It is also the terminology used in
userspace by libuuid and xfsprogs.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
[hch: remove now unused uuid.[ch]]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>