filefrag: avoid temporary buffer overflow

If an unknown flag is present in a FIEMAP extent, it is printed as a
hex value into a temporary buffer before adding it to the flags.  If
that unknown flag is over 0xfff then it will overflow the temporary
buffer.

Reported-by: Sarah Liu <wei3.liu@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-10335
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>