efi_loader: release file buffer after loading image

Commit 0e18f584 ("efi_loader: LoadImage: always allocate new pages")
ensured that whether we load an image from file or from memory we end up
with the same number of newly allocated buffers. But essentially we ended
up with one buffer too many in both cases:

efi_load_pe() copies and rebases the UEFI image.
We do not need the buffer with the file contents afterwards.

Fixes: 0e18f584 ("efi_loader: LoadImage: always allocate new pages")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>