efi/libstub/file: Merge file name buffers to reduce stack usage

Arnd reports that commit

  9302c1bb

 ("efi/libstub: Rewrite file I/O routine")

reworks the file I/O routines in a way that triggers the following
warning:

  drivers/firmware/efi/libstub/file.c:240:1: warning: the frame size
            of 1200 bytes is larger than 1024 bytes [-Wframe-larger-than=]

We can work around this issue dropping an instance of efi_char16_t[256]
from the stack frame, and reusing the 'filename' field of the file info
struct that we use to obtain file information from EFI (which contains
the file name even though we already know it since we used it to open
the file in the first place)

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200409130434.6736-8-ardb@kernel.org