mmp: don't assume NUL termination for MMP strings

mmp: don't assume NUL termination for MMP strings

Don't assume that mmp_nodename and mmp_bdevname are NUL terminated,
since very long node/device names may completely fill the buffers.

Limit string printing to the maximum buffer size for safety, and
change the field definitions to __u8 to make it more clear that
they are not NUL-terminated strings, as is done with other strings
in the superblock that do not have NUL termination.

Signed-off-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>