xfstests: allow dump file name to be passed as arg

xfstests: allow dump file name to be passed as arg

xfsdump tests using tapes (rather than files) can pass the tape pathname
to the dump/restore helper functions using the -f option. Change
_parse_args() so that this can be done for file-based tests as well, so
that they don't have to set the global 'dump_file' variable before doing
the dump or restore.

Signed-off-by: Bill Kendall <wkendall@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>