Extract xfs out into FSAL_XFS

FSAL_XFS shares most of it's code with FSAL_VFS, but is now an
independent FSAL. This solves some issues, and get's rid of the
need to specify the handle library path.

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>
Simplify vfs_stat_by_handle

Signed-off-by: Frank S. Filz <ffilzlnx@mindspring.com>