Add do_ftruncate that truncates a struct file

Add do_ftruncate that truncates a struct file

do_sys_ftruncate receives a file descriptor, fgets the struct file, and
finally actually truncates the file.

do_ftruncate allows for passing in a file directly, with the caller
already holding a reference to it.

Signed-off-by: Tony Solomonik <tony.solomonik@gmail.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20240202121724.17461-2-tony.solomonik@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>