Avoid symbol collision with in-kernel zstdlib

For Linux, when zfs is compiled as an in kernel static variant
and the in kernel zstd library is compiled statically into the kernel
a symbol collision will occur.  This wrapper header renames all
of the relevant zstd functions to avoid this problem.

Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Closes #10775

include/sys/zstd/zstd.h

@@ -83,13 +83,13 @@ typedef struct zfs_zstd_header {
 int zstd_init(void);
 void zstd_fini(void);
 
-size_t zstd_compress(void *s_start, void *d_start, size_t s_len, size_t d_len,
-    int level);
-int zstd_get_level(void *s_start, size_t s_len, uint8_t *level);
-int zstd_decompress_level(void *s_start, void *d_start, size_t s_len,
+size_t zfs_zstd_compress(void *s_start, void *d_start, size_t s_len,
+    size_t d_len, int level);
+int zfs_zstd_get_level(void *s_start, size_t s_len, uint8_t *level);
+int zfs_zstd_decompress_level(void *s_start, void *d_start, size_t s_len,
     size_t d_len, uint8_t *level);
-int zstd_decompress(void *s_start, void *d_start, size_t s_len, size_t d_len,
-    int n);
+int zfs_zstd_decompress(void *s_start, void *d_start, size_t s_len,
+    size_t d_len, int n);
 
 #ifdef	__cplusplus
 }