Fix incorrect type in fs_read and write

src/fs_read.hpp

@@ -32,7 +32,7 @@ namespace fs
        void         *buf_,
        const size_t  count_)
   {
-    int rv;
+    ssize_t rv;
 
     rv = ::read(fd_,buf_,count_);
 

src/fs_write.hpp

@@ -32,7 +32,7 @@ namespace fs
         const void   *buf_,
         const size_t  count_)
   {
-    int rv;
+    ssize_t rv;
 
     rv = ::write(fd_,buf_,count_);