fix: file: Rename fcheck lookup_fd_rcu (v5.10.220)

fix: file: Rename fcheck lookup_fd_rcu (v5.10.220)

See upstream backported commit:

  commit c4716bb296504cbc64aeefb370df44e821214c44
  Author: Eric W. Biederman <ebiederm@xmission.com>
  Date:   Fri Nov 20 17:14:27 2020 -0600

    file: Rename fcheck lookup_fd_rcu

    [ Upstream commit 460b4f812a9d473d4b39d87d37844f9fc30a9eb3 ]

    Also remove the confusing comment about checking if a fd exists.  I
    could not find one instance in the entire kernel that still matches
    the description or the reason for the name fcheck.

    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/20201120231441.29911-10-ebiederm@xmission.com
Change-Id: Ib880bd8feef1c5d75d2a018cd93a1d464485ab7b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>