vdev_file: unify FreeBSD and Linux implementations (#17046)

Kernel & userspace specifics are in zfs_file_os.c, so there's no
particular reason these have to be separate.

The one platform-specific part is in the Linux kernel part, to offload
flushes to a taskq if we're already inside a filesystem transaction.
This would be normally be an unsatisfying wart, but I'm intending to
remove this shortly, so I'm content to leave it gated for the moment.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

Signed-off-by: Rob Norris <rob.norris@klarasystems.com>

module/Makefile.bsd

@@ -199,7 +199,6 @@ SRCS+=	abd_os.c \
 	kmod_core.c \
 	spa_os.c \
 	sysctl_os.c \
-	vdev_file.c \
 	vdev_geom.c \
 	vdev_label_os.c \
 	zfs_acl.c \
@@ -313,6 +312,7 @@ SRCS+=	abd.c \
 	vdev.c \
 	vdev_draid.c \
 	vdev_draid_rand.c \
+	vdev_file.c \
 	vdev_indirect_births.c \
 	vdev_indirect.c \
 	vdev_indirect_mapping.c \