fs/9p: avoid debug OOPS when reading a long symlink

Reading a symlink longer than the given buffer, a p9_debug use would
try to print the link name (not NUL-terminated) using a %s format.
Use %.*s instead, and replace the strncpy+strnlen with functionally
equivalent strlen+memcpy.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>