ext4: validate s_reserved_gdt_blocks on mount

ext4: validate s_reserved_gdt_blocks on mount

commit 5b9554dc5bf008ae7f68a52e3d7e76c0920938a2 upstream.

If s_reserved_gdt_blocks is extremely large, it's possible for
ext4_init_block_bitmap(), which is called when ext4 sets up an
uninitialized block bitmap, to corrupt random kernel memory.  Add the
same checks which e2fsck has --- it must never be larger than
blocksize / sizeof(__u32) --- and then add a backup check in
ext4_init_block_bitmap() in case the superblock gets modified after
the file system is mounted.

Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.2:
 - Drop the second check in ext4_init_block_bitmap() since it can't return
   an error code
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>