libmultipath: Don't chop const strings

multipath was chopping sysfs attr strings in order to make sure that
ending whitespace didn't cause them to overflow their buffer. However
those strings were const strings. This patch makes multipath check how
much of the string length is ending whitespace, so that it can tell if
it will really overflow the buffer without chopping the const string.

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>