xfs_db: redirect printfs when metadumping to stdout

xfs_db: redirect printfs when metadumping to stdout

If we're metadumping to stdout, we don't want xfs_db's various dbprintf
statements dumping to stdout because that'll corrupt the metadump.
Therefore, let outf point to the existing stdout and redirect stdout to
stderr for the duration of the dump operation.

Reported-by: David Shaw <dshaw@jabberwocky.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>