Zstd: rework ZSTD_isError symbol renaming

The import of Zstd v1.5.7 in a2ac9cd606
added an unconditional renaming of ZSTD_isError to zfs_ZSTD_isError
with an asm directive.  Instead, do it with a define that is conditioned
on whether zstd_compat_wrapper.h is actually in use.  Also add a define
to that header so that it can be detected.  This allows the build to
work without using the compat wrapper.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Libby <rlibby@FreeBSD.org>
Closes #18483

module/zstd/include/zstd_compat_wrapper.h

@@ -33,6 +33,8 @@
  * Copyright (c) 2020, Sebastian Gottschall
  */
 
+#define	_ZSTD_COMPAT_WRAPPER_H
+
 /*
  * This wrapper fixes a problem, in case the ZFS filesystem driver, is compiled
  * statically into the kernel.

module/zstd/lib/common/zstd_common.c

@@ -9,7 +9,9 @@
  * You may select, at your option, one of the above-listed licenses.
  */
 
-
+#ifdef _ZSTD_COMPAT_WRAPPER_H
+#undef ZSTD_isError   /* defined within zstd_internal.h */
+#endif
 
 /*-*************************************
 *  Dependencies
@@ -34,7 +36,9 @@ const char* ZSTD_versionString(void) { return ZSTD_VERSION_STRING; }
 /*! ZSTD_isError() :
  *  tells if a return value is an error code
  *  symbol is required for external callers */
-unsigned ZSTD_isError(size_t code) __asm__("zfs_ZSTD_isError");
+#ifdef _ZSTD_COMPAT_WRAPPER_H
+#define	ZSTD_isError zfs_ZSTD_isError
+#endif
 unsigned ZSTD_isError(size_t code) { return ERR_isError(code); }
 
 /*! ZSTD_getErrorName() :