tools/blktap, blktap2: include <sys/mount.h> instead of <linux/fs.h>

tools/blktap, blktap2: include <sys/mount.h> instead of <linux/fs.h>

The former is a userspace sanitised header which contains the
definitions we need. In some distros linux/fs.h defines WRITE which
conflicts with blktaps own use of that name.

Also there is no reason to use <linux/errno.h> over the more normal
<errno.h>.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>