gfs2: Get rid of gfs2_ea_strlen

Function gfs2_ea_strlen is only called from ea_list_i, so inline it
there.  Remove the duplicate switch statement and the creative use of
memcpy to set a null byte.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>
Reviewed-by: Bob Peterson <rpeterso@redhat.com>