libblkid: (dasd) add buffer size guards to helper functions

libblkid: (dasd) add buffer size guards to helper functions

Add explicit buffer size parameters to dasd_get_volser(),
dasd_get_dsnam(), is_dasd_cdl_label(), is_dasd_ldl_label()
and is_dasd_f4_label() so that these functions can protect
against buffer overflows without relying on callers to pass
correctly sized buffers.

The conversion functions still follow the DASD standard field
sizes (DASD_VOLSER_LENGTH, sizeof DS1DSNAM), but clamp to the
buffer size as a safety net. The label detection functions
derive their minimum sizes from the actual struct offsets and
magic constants rather than hardcoded numbers.

Signed-off-by: Karel Zak <kzak@redhat.com>