MFC r225338:

Fix the check in dircheck() on namlen.

The value of namlen is copied from on-disk d_namlen, which is a 8-bit
unsigned integer which can never exceed MAXNAMLEN (255) so the test is
always true. Moreover, UFS does not allow d_namelen being zero.

Change namlen from u_int to u_int8_t, and replace the unneeded test
with a useful test.

PR: bin/160339
Submitted by: Eugene Grosbein <eugen grosbein.pp.ru>