fs usage: fix block size returned on older Linux 2.6.
* src/filemanager/mountlist.c: Fall back to (struct statfs).f_frsize
which is available since Linux 2.6.
* m4.include/fsusage.m4 (STAT_STATFS2_FRSIZE): Always define
when the member is available so it can be used as a fallback.
Signed-off-by: Andrew Borodin <aborodin@vmail.ru>