zcommon: Refactor FPU state handling in fletcher4

Currently calls to kfpu_begin() and kfpu_end() are split between
the init() and fini() functions of the particular SIMD
implementation. This was done in #14247 as an optimization measure
for the ABD adapter. Unfortunately the split complicates FPU
handling on platforms that use a local FPU state buffer, like
Windows and macOS.

To ease porting, we introduce a boolean struct member in
fletcher_4_ops_t, indicating use of the FPU, and move the FPU state
handling from the SIMD implementations to the call sites.

Reviewed-by: Tino Reichardt <milky-zfs@mcmilk.de>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Attila Fülöp <attila@fueloep.org>
Closes #14600

module/zcommon/zfs_fletcher_intel.c

@@ -51,7 +51,6 @@ ZFS_NO_SANITIZE_UNDEFINED
 static void
 fletcher_4_avx2_init(fletcher_4_ctx_t *ctx)
 {
-	kfpu_begin();
 	memset(ctx->avx, 0, 4 * sizeof (zfs_fletcher_avx_t));
 }
 
@@ -82,7 +81,6 @@ fletcher_4_avx2_fini(fletcher_4_ctx_t *ctx, zio_cksum_t *zcp)
 	    64 * ctx->avx[3].v[3];
 
 	ZIO_SET_CHECKSUM(zcp, A, B, C, D);
-	kfpu_end();
 }
 
 #define	FLETCHER_4_AVX2_RESTORE_CTX(ctx)				\
@@ -163,6 +161,7 @@ const fletcher_4_ops_t fletcher_4_avx2_ops = {
 	.fini_byteswap = fletcher_4_avx2_fini,
 	.compute_byteswap = fletcher_4_avx2_byteswap,
 	.valid = fletcher_4_avx2_valid,
+	.uses_fpu = B_TRUE,
 	.name = "avx2"
 };