dbuf_hold_impl() cleanup to improve cached read performance

dbuf_hold_impl() cleanup to improve cached read performance

Currently every dbuf_hold_impl() incurs kmem_alloc() and kmem_free()
which can be costly for cached read performance.

This change reverts the dbuf_hold_impl() fix stack commit, i.e.
fc5bb51f08a6c91ff9ad3559d0266eeeab0b1f61 to eliminate the extra
kmem_alloc() and kmem_free() operations and improve cached read
performance. With the change, each dbuf_hold_impl() frame uses 40 bytes
more, total of 800 for 20 recursive levels. Linux kernel stack sizes are
8K and 16K for 32bit and 64bit, respectively, so stack overrun risk is
limited.

Sample stack output comparisons with 50 PB file and recordsize=512
Current code
 11)     2240      64   arc_alloc_buf+0x4a/0xd0 [zfs]
 12)     2176     264   dbuf_read_impl.constprop.16+0x2e3/0x7f0 [zfs]
 13)     1912     120   dbuf_read+0xe5/0x520 [zfs]
 14)     1792      56   dbuf_hold_impl_arg+0x572/0x630 [zfs]
 15)     1736      64   dbuf_hold_impl_arg+0x508/0x630 [zfs]
 16)     1672      64   dbuf_hold_impl_arg+0x508/0x630 [zfs]
 17)     1608      40   dbuf_hold_impl+0x23/0x40 [zfs]
 18)     1568      40   dbuf_hold_level+0x32/0x60 [zfs]
 19)     1528      16   dbuf_hold+0x16/0x20 [zfs]

dbuf_hold_impl() cleanup
 11)     2320      64   arc_alloc_buf+0x4a/0xd0 [zfs]
 12)     2256     264   dbuf_read_impl.constprop.17+0x2e3/0x7f0 [zfs]
 13)     1992     120   dbuf_read+0xe5/0x520 [zfs]
 14)     1872      96   dbuf_hold_impl+0x50f/0x5e0 [zfs]
 15)     1776     104   dbuf_hold_impl+0x4df/0x5e0 [zfs]
 16)     1672     104   dbuf_hold_impl+0x4df/0x5e0 [zfs]
 17)     1568      40   dbuf_hold_level+0x32/0x60 [zfs]
 18)     1528      16   dbuf_hold+0x16/0x20 [zfs]

Performance observations on 8K recordsize filesystem:
- 8/128/1024K at 1-128 sequential cached read, ~3% improvement

Testing done on Ubuntu 18.04 with 4.15 kernel, 8vCPUs and SSD storage on
VMware ESX.

Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Nguyen <tony.nguyen@delphix.com>
Closes #9351