eventpoll: rename attach_epitem() to ep_attach_file()

eventpoll: rename attach_epitem() to ep_attach_file()

ep_remove_file() tears down the f_ep linkage that attach_epitem()
establishes, so the pair should look like one. Rename to
ep_attach_file() for the "ep_*" + subject symmetry and to match the
naming used elsewhere in the file (ep_insert, ep_modify, ep_remove,
ep_remove_file, ep_remove_epi, ep_unregister_pollwait).

Pure rename; no functional change.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Link: https://patch.msgid.link/20260424-work-epoll-rework-v1-8-249ed00a20f3@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>