client: allow unprivileged users to clear suid/sgid

client: allow unprivileged users to clear suid/sgid

The fallocate path will try to clear the suid/sgid if a unprivileged
user changed the file. And the VFS will set the ATTR_MODE and at the
same time by clearing the suid/sgid mode bits.

There is no Posix item requires that we should clear the suid/sgid
in fallocate code path but this is the default behaviour for most of
the filesystems and the VFS layer. And also the same for the write
code path, which have already support it.

Fixes: https://tracker.ceph.com/issues/58680
Signed-off-by: Xiubo Li <xiubli@redhat.com>