dm-verity: increase sprintf buffer size

dm-verity: increase sprintf buffer size

The prefix "DM_VERITY_ERR_BLOCK_NR" is 22 chars. Add '=', one digit for
type, ',', up to 20 digits for a u64 block number, and a NUL terminator:
that's 46 bytes. The buffer is 42 bytes. For block numbers >= 16 decimal
digits (devices larger than ~16 EB with 4K blocks), snprintf silently
truncates the uevent environment variable.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Assisted-by: Claude:claude-opus-4.6
Fixes: 65ff5b7ddf05 ("dm verity: add error handling modes for corrupted blocks")
Cc: stable@vger.kernel.org