perf symbols: Check if we read regular file in dso__load()

The current code in dso__load() calls is_regular_file(), but it checks
its return value only after calling symsrc__init().

That can make symsrc__init() block in elf_* functions on reading
the file if the file happens to be device and not regular one.

Call symsrc__init() only for regular files. Also remove the
symsrc__destroy() cleanup, which is not needed now, because we call
symsrc__init() only for regular files.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180215122635.24029-3-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>