ext2: Zero our b_size in ext2_quota_read()

ext2_quota_read() doesn't initialize tmp_bh.b_size before calling
ext2_get_block() where we access it. Since it is a local variable it
might contain some garbage. Make sure it is filled with reasonable
value before passing.

Signed-off-by: Manish Katiyar <mkatiyar@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>