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/Kbuild.in

@@ -385,6 +385,7 @@ ZFS_OBJS := \
 	vdev.o \
 	vdev_draid.o \
 	vdev_draid_rand.o \
+	vdev_file.o \
 	vdev_indirect.o \
 	vdev_indirect_births.o \
 	vdev_indirect_mapping.o \
@@ -446,7 +447,6 @@ ZFS_OBJS_OS := \
 	spa_misc_os.o \
 	trace.o \
 	vdev_disk.o \
-	vdev_file.o \
 	vdev_raidz.o \
 	vdev_label_os.o \
 	zfs_acl.o \