filelock: fl_pid field should be signed int

This field has been unsigned for a very long time, but most users of the
struct file_lock and the file locking internals themselves treat it as a
signed value. Change it to be pid_t (which is a signed int).

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/r/20240131-flsplit-v3-1-c6129007ee8d@kernel.org


Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>