ext4: fix fencepost in s_first_meta_bg validation

It is OK for s_first_meta_bg to be equal to the number of block group
descriptor blocks.  (It rarely happens, but it shouldn't cause any
problems.)

https://bugzilla.kernel.org/show_bug.cgi?id=194567



Fixes: 3a4b77cd
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@vger.kernel.org