LU-14428 libcfs: discard cfs_trace_allocate_string_buffer

LU-14428 libcfs: discard cfs_trace_allocate_string_buffer

cfs_trace_allocate_string_buffer() is now only used once, and it is a
simple wrapper for kmalloc().  So discard it and use kmalloc directly.

Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I4658f58d3e073be2092e0af1de0d6ecec15da6a6
Reviewed-on: https://review.whamcloud.com/41491
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: Aurelien Degremont <degremoa@amazon.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>