exec: binfmt_misc: kill the onstack iname[BINPRM_BUF_SIZE] array

exec: binfmt_misc: kill the onstack iname[BINPRM_BUF_SIZE] array

After the previous change "fmt" can't go away, we can kill
iname/iname_addr and use fmt->interpreter.

Link: http://lkml.kernel.org/r/20170922143653.GA17232@redhat.com
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ben Woodard <woodard@redhat.com>
Cc: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Jim Foraker <foraker1@llnl.gov>
Cc: <tdhooge@llnl.gov>
Cc: Travis Gummels <tgummels@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>