Linux 5.11 compat: x86 SIMD: fix kernel_fpu_{begin,end}() detection

Linux 5.11 changed kernel_fpu_begin() to an inlined function and
moved the functionality to kernel_fpu_begin_mask(). This breaks the
existing detection mechanism since it checks if kernel_fpu_begin is
an exported kernel symbol, which isn't the case for an inlined
function.

To avoid assumptions about internal implementation, replace
ZFS_LINUX_TEST_RESULT_SYMBOL in favor of ZFS_LINUX_TEST_RESULT
which already makes sure kernel_fpu_{begin,end}() is usable by us.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #13147