Fix gcc warning in kfpu_begin()

Observed when building on CentOS 8 Stream. Remove the out
label at the end of the function and instead return.

  linux/simd_x86.h: In function 'kfpu_begin':
  linux/simd_x86.h:337:1: error: label at end of compound statement

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Reviewed-by: Attila Fülöp <attila@fueloep.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13089