Fix HAVE_FS_STRUCT_SPINLOCK check for gcc-4.1.2

Older versions of gcc (gcc-4.1.2) will treat an 'incompatible
pointer type' as a warning instead of an error. This results
in HAVE_FS_STRUCT_SPINLOCK being defined incorrectly. This
failure mode was observed when using a RHEL6 2.6.32 based kernel
under RHEL5.5 which contains the old version of gcc. To resolve
the issue the warning is explicitly promoted to an error.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>