mkfs: reduce internal log size when log stripe units are in play

mkfs: reduce internal log size when log stripe units are in play

Currently, one can feed mkfs a combination of options like this:

$ truncate -s 6366g /tmp/a ; mkfs.xfs -f /tmp/a -d agcount=3200 -d su=256k,sw=4
meta-data=/tmp/a                 isize=512    agcount=3200, agsize=521536 blks
         =                       sectsz=512   attr=2, projid32bit=1
         =                       crc=1        finobt=1, sparse=1, rmapbt=0
         =                       reflink=1    bigtime=0 inobtcount=0
data     =                       bsize=4096   blocks=1668808704, imaxpct=5
         =                       sunit=64     swidth=256 blks
naming   =version 2              bsize=4096   ascii-ci=0, ftype=1
log      =internal log           bsize=4096   blocks=521536, version=2
         =                       sectsz=512   sunit=64 blks, lazy-count=1
realtime =none                   extsz=4096   blocks=0, rtextents=0
Metadata corruption detected at 0x55e88052c6b6, xfs_agf block 0x1/0x200
libxfs_bwrite: write verifier failed on xfs_agf bno 0x1/0x1
mkfs.xfs: writing AG headers failed, err=117

The format fails because the internal log size sizing algorithm
specifies a log size of 521492 blocks to avoid taking all the space in
the AG, but align_log_size sees the stripe unit and rounds that up to
the next stripe unit, which is 521536 blocks.

Fix this problem by rounding the log size down if rounding up would
result in a log that consumes more space in the AG than we allow.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>