linux: Fix __sem_check_add_mapping name length

Take in consideration the trailling NULL since sem_search uses
strcmp to compare entries.

Checked on x86_64-linux-gnu and powerpc-linux-gnu (where it triggered
a nptl/tst-sem7 regression).

sysdeps/pthread/sem_routines.c

@@ -72,6 +72,7 @@ __sem_check_add_mapping (const char *name, int fd, sem_t *existing)
   size_t namelen = strlen (name);
   if (namelen > NAME_MAX)
     return SEM_FAILED;
+  namelen += 1;
 
   sem_t *result = SEM_FAILED;