This change renames fcheck_files to files_lookup_fd_rcu.  All of the
remaining callers take the rcu_read_lock before calling this function
so the _rcu suffix is appropriate.  This change also tightens up the
debug check to verify that all callers hold the rcu_read_lock.

All callers that used to call files_check with the files->file_lock
held have now been changed to call files_lookup_fd_locked.

This change of name has helped remind me of which locks and which
guarantees are in place helping me to catch bugs later in the
patchset.

The need for better names became apparent in the last round of
discussion of this set of changes[1].

[1] https://lkml.kernel.org/r/CAHk-=wj8BQbgJFLa+J0e=iT-1qpmCRTbPAJ8gd6MJQ=kbRPqyQ@mail.gmail.com
Link: https://lkml.kernel.org/r/20201120231441.29911-9-ebiederm@xmission.comSigned-off-by: NEric W. Biederman <ebiederm@xmission.com>