Improve fstat(2) performance

Improve fstat(2) performance

There is at most a factor of 3x performance improvement to be
had by using the Linux generic_fillattr() helper.  However, to
use it safely we need to ensure the values in a cached inode
are kept rigerously up to date.  Unfortunately, this isn't
the case for the blksize, blocks, and atime fields.  At the
moment the authoritative values are still stored in the znode.

This patch introduces an optimized zfs_getattr_fast() call.
The idea is to use the up to date values from the inode and
the blksize, block, and atime fields from the znode.  At some
latter date we should be able to strictly use the inode values
and further improve performance.

The remaining overhead in the zfs_getattr_fast() call can be
attributed to having to take the znode mutex.  This overhead is
unavoidable until the inode is kept strictly up to date.  The
the careful reader will notice the we do not use the customary
ZFS_ENTER()/ZFS_EXIT() macros.  These macro's are designed to
ensure the filesystem is not torn down in the middle of an
operation.  However, in this case the VFS is holding a
reference on the active inode so we know this is impossible.

=================== Performance Tests ========================

This test calls the fstat(2) system call 10,000,000 times on
an open file description in a tight loop.  The test results
show the zfs stat(2) performance is now only 22% slower than
ext4.  This is a 2.5x improvement and there is a clear long
term plan to get to parity with ext4.

filesystem    | test-1  test-2  test-3  | average | times-ext4
--------------+-------------------------+---------+-----------
ext4          |  7.785s  7.899s  7.284s |  7.656s | 1.000x
zfs-0.6.0-rc4 | 24.052s 22.531s 23.857s | 23.480s | 3.066x
zfs-faststat  |  9.224s  9.398s  9.485s |  9.369s | 1.223x

The second test is to run 'du' of a copy of the /usr tree
which contains 110514 files.  The test is run multiple times
both using both a cold cache (/proc/sys/vm/drop_caches) and
a hot cache.  As expected this change signigicantly improved
the zfs hot cache performance and doesn't quite bring zfs to
parity with ext4.

A little surprisingly the zfs cold cache performance is better
than ext4.  This can probably be attributed to the zfs allocation
policy of co-locating all the meta data on disk which minimizes
seek times.  By default the ext4 allocator will spread the data
over the entire disk only co-locating each directory.

filesystem    | cold    | hot
--------------+---------+--------
ext4          | 13.318s | 1.040s
zfs-0.6.0-rc4 |  4.982s | 1.762s
zfs-faststat  |  4.933s | 1.345s