nfs: Make NFS_FSCACHE select NETFS_SUPPORT instead of depending on it

Having the NFS_FSCACHE option depend on the NETFS_SUPPORT options makes
selecting NFS_FSCACHE impossible unless another option that additionally
selects NETFS_SUPPORT is already selected.

As a result, for example, being able to reach and select the NFS_FSCACHE
option requires the CEPH_FS or CIFS option to be selected beforehand, which
obviously doesn't make much sense.

Let's correct this by making the NFS_FSCACHE option actually select the
NETFS_SUPPORT option, instead of depending on it.

Fixes: 915cd30c ("netfs, fscache: Combine fscache with netfs")
Cc: stable@vger.kernel.org
Reported-by: Diederik de Haas <didi.debian@cknow.org>
Signed-off-by: Dragan Simic <dsimic@manjaro.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>