Move definition of maximum file name length from <syscall> to file.h

src/lib/user/syscall.c

@@ -166,7 +166,7 @@ mkdir (const char *dir)
 }
 
 bool
-readdir (int fd, char name[READDIR_MAX_LEN + 1]) 
+readdir (int fd, char name[FNAME_MAX_LEN + 1]) 
 {
   return syscall2 (SYS_READDIR, fd, name);
 }