proc: fix missing pde_set_flags() for net proc files

proc: fix missing pde_set_flags() for net proc files

To avoid potential UAF issues during module removal races, we use
pde_set_flags() to save proc_ops flags in PDE itself before
proc_register(), and then use pde_has_proc_*() helpers instead of directly
dereferencing pde->proc_ops->*.

However, the pde_set_flags() call was missing when creating net related
proc files.  This omission caused incorrect behavior which FMODE_LSEEK was
being cleared inappropriately in proc_reg_open() for net proc files.  Lars
reported it in this link[1].

Fix this by ensuring pde_set_flags() is called when register proc entry,
and add NULL check for proc_ops in pde_set_flags().

[wangzijie1@honor.com: stash pde->proc_ops in a local const variable, per Christian]
Link: https://lkml.kernel.org/r/20250821105806.1453833-1-wangzijie1@honor.com
Link: https://lkml.kernel.org/r/20250818123102.959595-1-wangzijie1@honor.com
Link: https://lore.kernel.org/all/20250815195616.64497967@chagall.paradoxon.rec/
Fixes: ff7ec8dc1b64 ("proc: use the same treatment to check proc_lseek as ones for proc_read_iter et.al")
Signed-off-by: wangzijie <wangzijie1@honor.com>
Reported-by: Lars Wendler <polynomial-c@gmx.de>
Tested-by: Stefano Brivio <sbrivio@redhat.com>
Tested-by: Petr Vaněk <pv@excello.cz>
Tested by: Lars Wendler <polynomial-c@gmx.de>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Kirill A. Shutemov <k.shutemov@gmail.com>
Cc: wangzijie <wangzijie1@honor.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>