libc: speed-up flockfile()/funlockfile()

For Honeycomb, we added proper file thread-safety for
all FILE* operations. However, we did implement that by
using an out-of-band hash table to map FILE* pointers
to phtread_mutex_t mutexes, because we couldn't change
the size of 'struct _sFILE' without breaking the ABI.

It turns out that our BSD-derived code already has
some support code to extend FILE* objects, so use it
instead. See libc/stdio/fileext.h

This patch gets rid of the hash table, and put the
mutex directly into the sFILE extension.

Change-Id: If1c3fe0a0a89da49c568e9a7560b7827737ff4d0

libc/stdio/findfp.c

@@ -58,7 +58,12 @@ static struct glue uglue = { 0, FOPEN_MAX - 3, usual };
 static struct glue *lastglue = &uglue;
 _THREAD_PRIVATE_MUTEX(__sfp_mutex);
 
-static struct __sfileext __sFext[3];
+static struct __sfileext __sFext[3] = {
+	_FILEEXT_INITIALIZER,
+	_FILEEXT_INITIALIZER,
+	_FILEEXT_INITIALIZER,
+};
+
 FILE __sF[3] = {
 	std(__SRD, STDIN_FILENO),		/* stdin */
 	std(__SWR, STDOUT_FILENO),		/* stdout */