xfs: fix integer overflow when validating extent size hints

xfs: fix integer overflow when validating extent size hints

Both file extent size hints are stored as 32-bit quantities, in units of
filesystem blocks.  As part of validating the hints, we convert these
quantities to bytes to ensure that the hint is congruent with the file's
allocation size.

The maximum possible hint value is 2097151 (aka XFS_MAX_BMBT_EXTLEN).
If the file allocation unit is larger than 2048, the unit conversion
will exceed 32 bits in size, which overflows the uint32_t used to store
the value used in the comparison.  This isn't a problem for files on the
data device since the hint will always be a multiple of the block size.
However, this is a problem for realtime files because the rtextent size
can be any integer number of fs blocks, and truncation of upper bits
changes the outcome of division.

Eliminate the overflow by performing the congruency check in units of
blocks, not bytes.  Otherwise, we get errors like this:

$ truncate -s 500T /tmp/a
$ mkfs.xfs -f -N /tmp/a -d extszinherit=2097151,rtinherit=1 -r extsize=28k
illegal extent size hint 2097151, must be less than 2097151 and a multiple of 7.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>