fs: make plain ->i_state access fail to compile

fs: make plain ->i_state access fail to compile

... to make sure all accesses are properly validated.

Merely renaming the var to __i_state still lets the compiler make the
following suggestion:
error: 'struct inode' has no member named 'i_state'; did you mean '__i_state'?

Unfortunately some people will add the __'s and call it a day.

In order to make it harder to mess up in this way, hide it behind a
struct. The resulting error message should be convincing in terms of
checking what to do:
error: invalid operands to binary & (have 'struct inode_state_flags' and 'int')

Of course people determined to do a plain access can still do it, but
nothing can be done for that case.

Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>