xenpmd.c: use dynamic allocation

xenpmd.c: use dynamic allocation

On NetBSD, d_name is larger than 256, so file_name[284] may not be large
enough (and gcc emits a format-truncation error).
Use asprintf() instead of snprintf() on a static on-stack buffer.

Signed-off-by: Manuel Bouyer <bouyer@netbsd.org>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>
Plus

define GNU_SOURCE for asprintf()

Harmless on NetBSD.

Signed-off-by: Manuel Bouyer <bouyer@netbsd.org>
Reviewed-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-Acked-by: Ian Jackson <iwj@xenproject.org>