[XFS] fix valid but harmless sparse warning

The new xlog_recover_do_reg_buffer checks call be16_to_cpu on di_gen which
is a 32bit value so sparse rightly complains. Fortunately the warning is
harmless because we don't care for the value, but only whether it's
non-NULL. Due to that fact we can simply kill the endian swaps on this and
the previous di_mode check entirely.

SGI-PV: 969656
SGI-Modid: xfs-linux-melb:xfs-kern:29709a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>