osdep/devmapper/getroot: Fix build error on 32-bit host

osdep/devmapper/getroot: Fix build error on 32-bit host

The gcc build has failed for 32-bit host (e.g. i386-emu and arm-emu)
due to mismatch between format specifier and data type.

../grub-core/osdep/devmapper/getroot.c: In function
'grub_util_pull_devmapper':

../grub-core/osdep/devmapper/getroot.c:265:75: error: format '%lu'
expects argument of type 'long unsigned int', but argument 2 has type
'int' [-Werror=format=]

../grub-core/osdep/devmapper/getroot.c:276:80: error: format '%lu'
expects argument of type 'long unsigned int', but argument 2 has type
'int' [-Werror=format=]

This patch fixes the problem by casting the type of calculated offset to
grub_size_t and use platform PRIuGRUB_SIZE as format specifier.

Signed-off-by: Michael Chang <mchang@suse.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>