xfs: remote attribute read too short

Reading a maximally size remote attribute fails when CRCs are
enabled with this verification error:

XFS (vdb): remote attribute header does not match required off/len/owner)

There are two reasons for this, the first being that the
length of the buffer being read is determined from the
args->rmtblkcnt which doesn't take into account CRC headers. Hence
the mapped length ends up being too short and so we need to
calculate it directly from the value length.

The second is that the byte count of valid data within a buffer is
capped by the length of the data and so doesn't take into account
that the buffer might be longer due to headers. Hence we need to
calculate the data space in the buffer first before calculating the
actual byte count of data.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>