osd_types: define max in eversion_t::max() to static

osd_types: define max in eversion_t::max() to static

max in eversion_t::max() can be static, no need to re constructs
it every time when calling eversion_t::max(), and max can be
constructed using eversion_t(epoch_t e, version_t v), it better than
firstly construct it by eversion_t() and then reassign it to (-1,-1)

Change-Id: I85de5685024e61ebc2d1816b0969be67bdaf45f5
Signed-off-by: yang.wang yang.wang@easystack.cn