Improve zfs_blkptr_verify()

[amotin]

• Skip config lock enter/exit for embedded blocks. They have no DVAs, so there is nothing to check under the lock.
• Skip CHECKSUM check and properly check PSIZE for embedded blocks.
• Add static branch predictions for unlikely conditions.
• Do not verify DVAs for blocks already in ARC. ARC hit already "verified" the first (often the only) DVA, and it does not worth to enter/exit config lock for nothing.

Some profiles show me up to 3% of CPU saving from this change.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.