md-multipath: Use seq_putc() in multipath_status()

A single character (closing square bracket) should be put into a sequence.
Thus use the corresponding function "seq_putc".

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Shaohua Li <sh.li@alibaba-inc.com>