fs/btrfs: Zero file data not backed by extents

Implicit holes in file data need to be zeroed explicitly, instead of
just leaving the data in the buffer uninitialized.

This led to kernels randomly failing to boot in "fun" ways when loaded
from btrfs with the no_holes feature enabled, because large blocks of
zeros in the kernel file contained random data instead.

Signed-off-by: Fabian Vogt <fvogt@suse.de>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>

grub-core/fs/btrfs.c

@@ -1603,6 +1603,8 @@ grub_btrfs_extent_read (struct grub_btrfs_data *data,
 	      csize = grub_le_to_cpu64 (key_out.offset) - pos;
 	      if (csize > len)
 		      csize = len;
+
+	      grub_memset (buf, 0, csize);
 	      buf += csize;
 	      pos += csize;
 	      len -= csize;