d/functions: Refine proc mounts entries traversal

Optimize the get_mnt_devno function, used twice in the cryptroot hook, to improve traversal efficiency. In cases with LDAP automount maps resulting in numerous entries in /proc/mounts, employing awk to filter out relevant mount points initially significantly reduces the hook's execution time, enhancing overall efficiency.

Signed-off-by: Chengen Du chengen.du@canonical.com