Store a decompression ID in 'struct kmod_path', so it can be later
stored in 'struct dso'.

Switch 'struct kmod_path's 'comp' from 'bool' to 'int' to return the
compressions array index. Add 0 index item into compressions array, so
that the comp usage stays as it was: 0 - no compression, != 0
compression index.

Update the kmod_path tests.

Committer notes:

Use a designated initializer + terminating comma, e.g. { .fmt = NULL, }, to fix
the build in several distros:

  centos:6:       util/dso.c:201: error: missing initializer
  centos:6:       util/dso.c:201: error: (near initialization for 'compressions[0].decompress')
  debian:9:       util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
  fedora:25:      util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
  fedora:26:      util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
  fedora:27:      util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
  oraclelinux:6:  util/dso.c:201: error: missing initializer
  oraclelinux:6:  util/dso.c:201: error: (near initialization for 'compressions[0].decompress')
  ubuntu:12.04.5: util/dso.c:201:2: error: missing initializer [-Werror=missing-field-initializers]
  ubuntu:12.04.5: util/dso.c:201:2: error: (near initialization for 'compressions[0].decompress') [-Werror=missing-field-initializers]
  ubuntu:16.04:   util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
  ubuntu:16.10:   util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
  ubuntu:16.10:   util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
  ubuntu:17.10:   util/dso.c:201:24: error: missing field 'decompress' initializer [-Werror,-Wmissing-field-initializers]
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180817094813.15086-7-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>