posix + fdtable: move zvfs_ftruncate() to fdtable.c

Move the zvfs_ftruncate() call from fs.c to fdtable.c, as file
types other than regular files can also be truncated.

Instead of hard-wiring zvfs_ftruncate() to fs_truncate(),
add a new ZVFS_IOCTL_TRUNCATE so that the operation can be
handled with other ioctl() operations.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>