mmp: abstract out repeated 'sizeof(buf), buf' usage

mmp: abstract out repeated 'sizeof(buf), buf' usage

The printf("%.*s") format requires both the buffer size and buffer
pointer to be specified for each use.  Since this is repeatedly given
as "(int)sizeof(buf), (char *)buf" for mmp_nodename and mmp_bdevname
fields, with typecasts to avoid compiler warnings.

Add a helper macro EXT2_LEN_STR() to avoid repeated boilerplate code.

This can also be used for other superblock buffer fields that may not
have NUL-terminated strings (e.g. s_volume_name, s_last_mounted,
s_{first,last}_error_func, s_mount_opts) to simplify code and avoid
the need for temporary buffers for NUL-termination.

Annotate the superblock string fields that may not be NUL-terminated.

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