smb3 client: add way to show directory leases for improved debugging

When looking at performance issues around directory caching, or debugging
directory lease issues, it is helpful to be able to display the current
directory leases (as we can e.g. or open files).  Create pseudo-file
/proc/fs/cifs/open_dirs that displays current directory leases.  Here
is sample output:

cat /proc/fs/cifs/open_dirs
 Version:1
 Format:
 <tree id> <sess id> <persistent fid> <path>
Num entries: 3
0xce4c1c68 0x7176aa54 0xd95ef58e     \dira      valid file info, valid dirents
0xce4c1c68 0x7176aa54 0xd031e211     \dir5      valid file info, valid dirents
0xce4c1c68 0x7176aa54 0x96533a90     \dir1      valid file info

Reviewed-by: Bharath SM <bharathsm@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>