bcachefs: Implement fileattr_(get|set)

bcachefs: Implement fileattr_(get|set)

inode_operations.fileattr_(get|set) didn't exist when the various flag
ioctls where implemented - but they do now, which means we can delete a
bunch of ioctl code in favor of standard VFS level wrappers.

Closes: https://lore.kernel.org/linux-bcachefs/7ltgrgqgfummyrlvw7hnfhnu42rfiamoq3lpcvrjnlyytldmzp@yazbhusnztqn/
Cc: Petr Vorel <pvorel@suse.cz>
Cc: Andrea Cervesato <andrea.cervesato@suse.de>
Cc: Dave Chinner <david@fromorbit.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>