Illumos 6288 - dmu_buf_will_dirty could be faster

6288 dmu_buf_will_dirty could be faster
Reviewed by: George Wilson <[email protected]>
Reviewed by: Paul Dagnelie <[email protected]>
Reviewed by: Justin Gibbs <[email protected]>
Reviewed by: Richard Elling <[email protected]>
Approved by: Robert Mustacchi <[email protected]>

References:
  https://www.illumos.org/issues/6288
  illumos/illumos-gate@0f2e7d0

Porting notes:
- [module/zfs/dbuf.c]
  - Fix 'warning: ISO C90 forbids mixed declarations and code'
    by moving 'dbuf_dirty_record_t *dr' to start of code block.

Ported-by: kernelOfTruth [email protected]
Signed-off-by: Brian Behlendorf <[email protected]>