perf symbols: Return the first entry with a given name in find_by_name method

When a dso contains multiple symbols which have same name, current
dso__find_symbol_by_name() only finds an one of them and there's no way
to get the all symbols without going through the rbtree.

So make symbols__find_by_name() return the first entry with the given
name and the next patch in this series will provide a way to iterate
from there, by the name ordered rb_tree, till a suitable symbol is
found.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Link: http://lkml.kernel.org/r/1421234288-22758-2-git-send-email-namhyung@kernel.org


[ Yanked this independent hunk, without changes, from a larger patch  ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>