Remove redundant m_path field

Contrary to the comment above its definition, the field m_path always
has the same value as e_path: the *only* modifications of m_path are all
of the form:

	strncpy(exp->m_export.m_path, exp->m_export.e_path,
		sizeof (exp->m_export.m_path) - 1);
	exp->m_export.m_path[sizeof (exp->m_export.m_path) - 1] = '\0';

So m_path is always just a copy of e_path.  In places where we need to
store a path to a submount of a CROSSMNT-exported filesystem, as in
cache.c, we just use a local variable.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Steve Dickson <steved@redhat.com>