xfs: remove l_iclog_heads

xfs: remove l_iclog_heads

l_iclog_heads is only used in one place and can be trivially derived
from l_iclog_hsize by a single shift operation.  Remove it, and switch
the initialization of l_iclog_hsize to use struct_size so that it is
directly derived from the on-disk format definition.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>