xfsprogs: restrict platform_test_xfs_fd to regular files

xfsprogs: restrict platform_test_xfs_fd to regular files

If a special file (block, char, pipe etc) resides on an
xfs filesystem, platform_test_xfs_[fd|path] will return
true, but a subsequent xfsctl will fail, because the file
operations to support the xfs ioctls are not set up on such
files (see i_fop assignments in xfs_setup_inode()).

>From the xfsctl manpage it's pretty clear that these functions
are supposed to return true if a subsequent xfsctl can be
handled, so it makes sense to exclude special files.

This was showing up in xfstest generic/306, which creates
the dev/null block device on an xfstest an tries to pwrite
to it with xfs_io - which emitted a warning when the xfsctl
trying to get geometry failed.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>