filefrag: fix block size value

filefrag: fix block size value

ioctl(FIGETBSZ) was used to get block size earlier but 2508eaa7
(filefrag: improvements to filefrag FIEMAP handling) moved to fstatfs
f_bsize which doesn't work well for many files systems.

Block size returned using fstatfs isn't block size but "optimal
transfer block size" as per man page.  Even stat st_blksize is
"preferred I/O block size" and in may file systems it may even vary
from file to file (POSIX).  This patch changes filefrag to use
FIGETBSZ preferentially over f_bsize.

[ Modified by tytso to add the fallback to f_bsize if FIGETBSZ fails
  for some reason ]

Signed-off-by: Rakesh Pandit <rakesh@tuxera.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>