filefrag: handle invalid st_dev and blksize cases

filefrag: handle invalid st_dev and blksize cases

It is possible to crash filefrag with a "Floating point exception" in
two different scenarios:

1. When fstat() returns a device ID set to 0
2. When FIGETBSZ ioctl returns a blocksize of 0

In both scenarios a divide-by-zero will occur in frag_report() because
variable blksize will be set to zero.

I've managed to trigger this crash with an old CephFS kernel client,
using xfstest generic/519.  The first scenario has been fixed by kernel
commit 75c9627efb72 ("ceph: map snapid to anonymous bdev ID").  The
second scenario is also fixed with commit 8f97d1e99149 ("vfs: fix
FIGETBSZ ioctl on an overlayfs file").

However, it is desirable to handle these two scenarios gracefully by
checking these conditions explicitly.

Signed-off-by: Luis Henriques <lhenriques@suse.de>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>