kernel_fpu fixes

This patch fixes a few issues when detecting which kernel_fpu functions
are available.

• Use kernel_fpu_begin() if it's exported on newer kernels.

• Use ZFS_LINUX_TRY_COMPILE_SYMBOL() to choose the right kernel_fpu function when using --enable-linux-builtin.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #8259
Closes #8363