__dsos__addnew should drop the constructor reference to dso after adding
it to the list, because __dsos__add() will get a reference that will be
kept while it is in the list.

This fixes DSO leaks when entries are removed to the list and the refcount
never gets to zero.

Refcnt debugger shows:
  ==== [0] ====
  Unreclaimed dso: 0x2fccab0
  Refcount +1 => 1 at
    ./perf(dso__new+0x1ff) [0x4a62df]
    ./perf(__dsos__addnew+0x29) [0x4a6e19]
    ./perf(dsos__findnew+0xd1) [0x4a7281]
    ./perf(machine__findnew_kernel+0x27) [0x4a5e17]
    ./perf() [0x4b8df2]
    ./perf(machine__create_kernel_maps+0x28) [0x4bb528]
    ./perf(machine__new_host+0xfa) [0x4bb84a]
    ./perf(init_probe_symbol_maps+0x93) [0x506713]
    ./perf() [0x455ffa]
    ./perf(cmd_probe+0x6c) [0x4566bc]
    ./perf() [0x47abc5]
    ./perf(main+0x610) [0x421f90]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f46df132af5]
    ./perf() [0x4220a9]
  Refcount +1 => 2 at
    ./perf(__dsos__addnew+0xfb) [0x4a6eeb]
    ./perf(dsos__findnew+0xd1) [0x4a7281]
    ./perf(machine__findnew_kernel+0x27) [0x4a5e17]
    ./perf() [0x4b8df2]
    ./perf(machine__create_kernel_maps+0x28) [0x4bb528]
    ./perf(machine__new_host+0xfa) [0x4bb84a]
    ./perf(init_probe_symbol_maps+0x93) [0x506713]
    ./perf() [0x455ffa]
    ./perf(cmd_probe+0x6c) [0x4566bc]
    ./perf() [0x47abc5]
    ./perf(main+0x610) [0x421f90]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f46df132af5]
    ./perf() [0x4220a9]
  Refcount +1 => 3 at
    ./perf(dsos__findnew+0x7e) [0x4a722e]
    ./perf(machine__findnew_kernel+0x27) [0x4a5e17]
    ./perf() [0x4b8df2]
    ./perf(machine__create_kernel_maps+0x28) [0x4bb528]
    ./perf(machine__new_host+0xfa) [0x4bb84a]
    ./perf(init_probe_symbol_maps+0x93) [0x506713]
    ./perf() [0x455ffa]
    ./perf(cmd_probe+0x6c) [0x4566bc]
    ./perf() [0x47abc5]
    ./perf(main+0x610) [0x421f90]
    /lib64/libc.so.6(__libc_start_main+0xf5) [0x7f46df132af5]
    ./perf() [0x4220a9]
  [snip]
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20151118064031.30709.81460.stgit@localhost.localdomainSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>