The redirection of mmap through __syscall(2) is broken in multiple ways.
Just call mmap directly.

Index: lib/libv4lconvert/libv4lsyscall-priv.h
--- lib/libv4lconvert/libv4lsyscall-priv.h.orig
+++ lib/libv4lconvert/libv4lsyscall-priv.h
@@ -107,10 +107,7 @@
 	syscall(SYS_mmap, (void *)(addr), (size_t)(len), \
 			(int)(prot), (int)(flags), (int)(fd), (off_t)(off))
 #elif defined(__OpenBSD__)
-register_t __syscall(quad_t, ...);
-#define SYS_MMAP(addr, len, prot, flags, fd, offset) \
-	__syscall((quad_t)SYS_mmap, (void *)(addr), (size_t)(len), \
-			(int)(prot), (int)(flags), (int)(fd), 0, (off_t)(offset))
+#define SYS_MMAP mmap
 #else
 #define SYS_MMAP(addr, len, prot, flags, fd, off) \
 	syscall(SYS_mmap2, (void *)(addr), (size_t)(len), \