Small tcache improvements

Change the tcache->counts[] entries to uint16_t - this removes
the limit set by char and allows a larger tcache.  Remove a few
redundant asserts.

bench-malloc-thread with 4 threads is ~15% faster on Cortex-A72.

Reviewed-by: DJ Delorie <dj@redhat.com>

	* malloc/malloc.c (MAX_TCACHE_COUNT): Increase to UINT16_MAX.
	(tcache_put): Remove redundant assert.
	(tcache_get): Remove redundant asserts.
	(__libc_malloc): Check tcache count is not zero.
	* manual/tunables.texi (glibc.malloc.tcache_count): Update maximum.

(cherry picked from commit 1f50f2ad)