Fix ARC stats for embedded blkptrs

Re-factor arc_read() to better account for embedded data blkptrs.
Previously, reading the payload from an embedded blkptr would cause
arcstats such as demand_metadata_misses to be bumped when there was
actually no cache "miss" because the data are already available in
the blkptr.

The following test procedure was used to demonstrate the problem:

   zpool create tank ...
   zfs create -o compression=lz4 tank/fs
   echo blah > /tank/fs/blah
   stat /tank/fs/blah
   grep 'meta.*mis' /proc/spl/kstat/zfs/arcstats

and repeating the last two steps to watch the metadata miss counter
increment.  This can also be demonstrated via the  zfs_arc_miss DTRACE4
probe in arc_read().

Reviewed-by: loli10K <[email protected]>
Reviewed-by: George Wilson <[email protected]>
Reviewed-by: Matt Ahrens <[email protected]>
Reviewed-by: Brian Behlendorf <[email protected]>
Reviewed-by: George Melikov <[email protected]>
Signed-off-by: Tim Chase <[email protected]>
Closes #8319 

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