FileStore: support compiling without libxfs

When configured with --without-libxfs, use GenericFileStoreBackend
instead of XfsFileStoreBackend for XFS.  At this point this would only
impact the allocation hint op.  The default, as before, is to compile
with --with-libxfs.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>