pointers are not castable to unsigned int

When printing a pointer's value for debugging purposes, use the
dedicated printf format specifier for pointers instead of assuming
that unsigned int ('x') is good enough.

Found by clang on FreeBSD 10.0.

Change-Id: I18c42df0bf03c2d0e9e7c757445b8ff0f616c671
Reviewed-on: http://gerrit.openafs.org/10820
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Perry Ruiter <pruiter@sinenomine.net>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>