xfs: fix overflow in xfs_growfs_data_private

In the case where growing a filesystem would leave the last AG
too small, the fixup code has an overflow in the calculation
of the new size with one fewer ag, because "nagcount" is a 32
bit number. If the new filesystem has > 2^32 blocks in it
this causes a problem resulting in an EINVAL return from growfs:

# xfs_io -f -c "truncate 19998630180864" fsfile
# mkfs.xfs -f -bsize=4096 -dagsize=76288719b,size=3905982455b fsfile
# mount -o loop fsfile /mnt
# xfs_growfs /mnt

meta-data=/dev/loop0 isize=256 agcount=52,
agsize=76288719 blks
= sectsz=512 attr=2
data = bsize=4096 blocks=3905982455, imaxpct=5
= sunit=0 swidth=0 blks
naming =version 2 bsize=4096 ascii-ci=0
log =internal bsize=4096 blocks=32768, version=2
= sectsz=512 sunit=0 blks, lazy-count=0
realtime =none extsz=4096 blocks=0, rtextents=0
xfs_growfs: XFS_IOC_FSGROWFSDATA xfsctl failed: Invalid argument

Reported-by: [e-mail]
Signed-off-by: Eric Sandeen <[e-mail]>
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Felix Blyakher <[e-mail]>
Signed-off-by: Felix Blyakher <[e-mail]>

fs/xfs/xfs_fsops.c

···
 	nagcount = new + (nb_mod != 0);
 	if (nb_mod && nb_mod < XFS_MIN_AG_BLOCKS) {
 		nagcount--;
-		nb = nagcount * mp->m_sb.sb_agblocks;
+		nb = (xfs_rfsblock_t)nagcount * mp->m_sb.sb_agblocks;
 		if (nb < mp->m_sb.sb_dblocks)
 			return XFS_ERROR(EINVAL);
 	}