staging: lustre: mdc: fix NULL pointer dereference in mdc_adjust_dirpages

The function mdc_adjust_dirpages is only called on platforms which
don't have pages 4K in size which is why kbuild only reported this
for platforms like the Alpha. The problem was a typo in ordering of
variables in the beginning of a while loop. We were accessing the
dp pointer before it was kmapped.

Signed-off-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>