warnings fix: sanitized 'write'alike callback prototype (int retval -> ssize_t)

Signed-off-by: Sergei Trofimovich <slyfox@inbox.ru>

vfs/direntry.c

@@ -834,7 +834,7 @@ vfs_s_read (void *fh, char *buffer, int count)
     return -1;
 }
 
-static int
+static ssize_t
 vfs_s_write (void *fh, const char *buffer, int count)
 {
     int n;