s3:smb2_server: only allocate the required buffer in the smb2 recvfile() code path

s3:smb2_server: only allocate the required buffer in the smb2 recvfile() code path

This way the buffer will likely be allocated within the existing talloc_pool,
which avoids one malloc() per request.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>