Squash error from geom by sizing ident strings to DISK_IDENT_SIZE.

Display attribute in future error strings and differentiate g_handleattr()
error messages for ease of debugging in the future.

"g_handleattr: md1 bio_length 24 strlen 31 -> EFAULT"

Reported by: swills
Reviewed by: imp cem avg
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D14962