zhold() wraps igrab() on Linux, and igrab() may fail when the inode
is in the process of being deleted. This means zhold() must only be
called when a reference exists and therefore it cannot be deleted.
This is the case for all existing consumers so add a VERIFY and a
comment explaining this requirement.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Adam Moss <c@yotes.com>
Closes#11704
1daad98176