objtool: Rename elf_read() to elf_open_read()

'struct elf *' handling is an open/close paradigm, make sure the naming
matches that:

   elf_open_read()
   elf_write()
   elf_close()

Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sami Tolvanen <samitolvanen@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200422103205.61900-3-mingo@kernel.org