libkmod: cache open file for later access

libkmod: cache open file for later access

If we are accessing several times the modules and reading some sections
by sucessive calls to the functions below, we are incurring in a penalty
of having to open, parse the header and close the file. For each
function.

	- kmod_module_get_info()
	- kmod_module_get_versions()
	- kmod_module_get_symbols()
	- kmod_module_get_dependency_symbols()

These functions are particularly important to depmod. It calls all of
them, for each module. Moreover there's a huge bottleneck in the open
operation if we are using compression. Every time we open the module we
need to uncompress the file and after getting the information we need we
discard the result. This is clearly shown by profiling depmod with perf
(record + report), using compressed modules:

 64.07%  depmod  libz.so.1.2.7       [.] 0x00000000000074b8                                            ◆
 18.18%  depmod  libz.so.1.2.7       [.] crc32                                                         ▒
  2.42%  depmod  libz.so.1.2.7       [.] inflate                                                       ▒
  1.17%  depmod  libc-2.16.so        [.] __memcpy_ssse3_back                                           ▒
  0.96%  depmod  [kernel.kallsyms]   [k] copy_user_generic_string                                      ▒
  0.89%  depmod  libc-2.16.so        [.] __strcmp_sse42                                                ▒
  0.82%  depmod  [kernel.kallsyms]   [k] hrtimer_interrupt                                             ▒
  0.77%  depmod  libc-2.16.so        [.] _int_malloc                                                   ▒
  0.44%  depmod  kmod-nolib          [.] kmod_elf_get_strings                                          ▒
  0.41%  depmod  kmod-nolib          [.] kmod_elf_get_dependency_symbols                               ▒
  0.37%  depmod  kmod-nolib          [.] kmod_elf_get_section                                          ▒
  0.36%  depmod  kmod-nolib          [.] kmod_elf_get_symbols
  ...

Average of running depmod 5 times, dropping caches between them, in a
slow spinning disk:

Before:   12.25 +- 0.20
After:     8.20 +- 0.21
m-i-t:     9.62 +- 0.27

So this patch leads to an improvement of ~33% over unpatched version,
ending up with 15% speedup over module-init-tools.