reiserfs_add_entry(): get rid of pointless namelen checks

In all cases namelen is ->d_name.len of some dentry; moreover, a dentry
that has passed ->lookup() without triggering ENAMETOOLONG check there.
The comment next to these checks is either a rudiment of some other
check that used to be there once upon a time, or an attempt to come up
with the possible reason for that check (well, more like "why does
ext3 do it?")

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>