Revert "io_uring: disable polling pollfree files"

This reverts commit fc78b2fc.

This breaks the Android api and for now, does not seem to be necessary
due to the lack of io_uring users in this kernel branch.  If io_uring
starts to be used more, it can be brought back in a ABI-safe way.

Bug: 161946584
Bug: 248008710
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I2696bd5e1ad61d3ab0e8d06f4ffe46718bb05845