pipe: introduce struct file_operations pipeanon_fops

So that fifos and anonymous pipes could have different f_op methods.
Preparation to simplify the next patch.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20250205181747.GB13817@redhat.com


Tested-by: K Prateek Nayak <kprateek.nayak@amd.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>