vn_stat: take into account va_blocksize when setting st_blksize

As currently st_blksize is always PAGE_SIZE, it is playing safe to not
use any smaller value.  For some cases this might not be optimal, but
at least nothing should get broken.

Generally I don't expect this commit to change much for the following
reasons (in case of VREG, VDIR):
- application I/O and physical I/O are sufficiently decoupled by
  filesystem code, buffer cache code, cluster and read-ahead logic
- not all applications use st_blksize as a hint, some use f_iosize, some
  use fixed block sizes

I expect writes to the middle of files on ZFS to benefit the most from
this change.

Silence from:	fs@
MFC after:	2 weeks

0 files changed, 0 insertions, 0 deletions