FreeBSD: Fix 12.2-STABLE after AT_BENEATH MFC

AT_BENEATH was merged to stable/12, where kern_unlinkat takes a
non-const path.  DECONST the path passed to kern_unlinkat in the
case where AT_BENEATH is defined.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #11139

module/os/freebsd/zfs/zfs_file_os.c

@@ -296,7 +296,8 @@ zfs_file_unlink(const char *fnamep)
 	rc = kern_funlinkat(curthread, AT_FDCWD, fnamep, FD_NONE, seg, 0, 0);
 #else
 #ifdef AT_BENEATH
-	rc = kern_unlinkat(curthread, AT_FDCWD, fnamep, seg, 0, 0);
+	rc = kern_unlinkat(curthread, AT_FDCWD, __DECONST(char *, fnamep),
+	    seg, 0, 0);
 #else
 	rc = kern_unlinkat(curthread, AT_FDCWD, __DECONST(char *, fnamep),
 	    seg, 0);