Add equivalents to FMODE_PREAD/PWRITE to VFS2.

This is mostly required for PipeTest_OffsetCalls.

The options are DenyPRead/PWrite rather than AllowPRead/PWrite since, in Linux
terms, fs/open.c:do_dentry_open sets FMODE_PREAD|FMODE_PWRITE unconditionally
(although it allows filesystem implementations of open to unset these flags),
so they're set for most FDs; it's usually FDs created outside of open(2) that
don't get them, e.g.:

- Syscall-created pipes (fs/pipe.c:create_pipe_files =>
  fs/file_table.c:alloc_file_pseudo)

- Epoll instances (fs/eventpoll.c:do_epoll_create =>
  fs/anon_inodes.c:anon_inode_getfile => alloc_file_pseudo)

- Sockets (net/socket.c:sock_alloc_file => alloc_file_pseudo)

This CL adds the flags to epoll instances; a subsequent CL reworks the VFS2
implementation of pipe FDs to be filesystem-independent and adds the flags
there, and sockets aren't implemented yet.

Updates #1035

PiperOrigin-RevId: 304506434

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