mke2fs: recalculate the reserved blocks when the last BG is dropped

mke2fs: recalculate the reserved blocks when the last BG is dropped

mke2fs -m option can set reserved blocks ratio up to 50%.  But if the
last block group is not big enough to support the necessary data
structures, it gets dropped, we have to recalculate the number of
reserved blocks so that the reserved blocks matches the requested
percentage.

It also avoids a problem where if the user specifies a reserved blocks
of 50%, and after the last partial block group was dropped, if the
number of reserved blocks is greater than 50%, e2fsck will complain.

Steps to reproduce:

1. Create a FS which has the overhead for the last BG
   and specify 50 % for reserved blocks ratio
 # mke2fs -m 50 -t ext4 DEV 1025M

 mke2fs 1.42.5 (29-Jul-2012)
 warning: 256 blocks unused.

 Filesystem label=
 OS type: Linux
 Block size=4096 (log=2)
 Fragment size=4096 (log=2)
 Stride=0 blocks, Stripe width=0 blocks
 656640 inodes, 2621440 blocks
 1310848 blocks (50.00%) reserved for the super user
 ~~~~~~~ <-- Reserved blocks exceed 50% of FS blocks count!

2. e2fsck outputs filesystem corruption
 # e2fsck DEV

 e2fsck 1.42.5 (29-Jul-2012)
 Corruption found in superblock.  (r_blocks_count = 1310848).

 The superblock could not be read or does not describe a correct ext2
 filesystem.  If the device is valid and it really contains an ext2
 filesystem (and not swap or ufs or something else), then the superblock
 is corrupt, and you might try running e2fsck with an alternate superblock:
     e2fsck -b 32768 <device>

Signed-off-by: Akira Fujita <a-fujita@rs.jp.ne.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>