fuse: always revalidate if exclusive create

fuse: always revalidate if exclusive create

commit df8629af293493757beccac2d3168fe5a315636e upstream.

Failure to do so may result in EEXIST even if the file only exists in the
cache and not in the filesystem.

The atomic nature of O_EXCL mandates that the cached state should be
ignored and existence verified anew.

Reported-by: Ken Schalk <kschalk@nvidia.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Wu Bo <bo.wu@vivo.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>