benchtests: Clean up the alloc_bufs

benchtests: Clean up the alloc_bufs

Drop realloc_bufs in favour of making alloc_bufs transparently
reallocate the buffers if it had allocated before.  Also consolidate
computation of buffer lengths so that they don't get repeated on every
reallocation.

	* benchtests/bench-string.h (buf1_size, buf2_size): New
	variables.
	(init_sizes): New function.
	(test_init): Use it.
	(alloc_buf, exit_error): New functions.
	(alloc_bufs): Use ALLOC_BUF.
	(realloc_bufs): Remove.
	* benchtests/bench-memcmp.c (do_test): Adjust.
	* benchtests/bench-memset-large.c (do_test): Likewise.
	* benchtests/bench-memset-walk.c (do_test): Likewise.
	* benchtests/bench-memset.c (do_test): Likewise.
	* benchtests/bench-strncmp.c (do_test): Likewise.