xfs: use tr_qm_dqalloc log reservation for dquot alloc

The dquot allocation path in xfs_qm_dqread() currently uses the
attribute set log reservation, which appears to be incorrect. We
have reports of transaction reservation overruns with the current
code. E.g., a repeated run of xfstests test generic/270 on a 512b
block size fs occassionally produces the following in dmesg:

	XFS (sdN): xlog_write: reservation summary:
	  trans type  = QM_DQALLOC (30)
	  unit res    = 7080 bytes
	  current res = -632 bytes
	  total reg   = 0 bytes (o/flow = 0 bytes)
	  ophdrs      = 0 (ophdr space = 0 bytes)
	  ophdr + reg = 0 bytes
	  num regions = 0

	XFS (sdN): xlog_write: reservation ran out. Need to up reservation

The dquot allocation case should consist of a write reservation
(i.e., we are allocating a range of the internal quota file) plus
the size of the actual dquots. We already have a log reservation
definition for this operation (tr_qm_dqalloc). Use it in
xfs_qm_dqread() and update the log reservation calculation function
to use the write res. calculation function rather than reading the
assumed to be pre-calculated value directly.

Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>