mdadm: use standard libc nftw

mdadm: use standard libc nftw

commit bd648e3bec3d ("mdadm: Remove klibc and uclibc support") removes
macro HAVE_NFTW/HAVE_FTW and uses libc header ftw.h. But it leaves the
codes in lib.c which let mdadm command call nftw defined in lib.c. It
needs to remove these codes.

The bug can be reproduced by:
mdadm -CR /dev/md0 --level raid5 --metadata=1.1 --chunk=32 --raid-disks 3
--size 10000 /dev/loop1 /dev/loop2 /dev/loop3
mdadm /dev/md0 --grow --chunk=64
mdadm: /dev/md0: cannot open component -unknown-

Fixes: bd648e3bec3d ("mdadm: Remove klibc and uclibc support")
Signed-off-by: Xiao Ni <xni@redhat.com>