udf_readdir: do not advance offset if entry can not be uio-ed
udf_readdir: do not advance offset if entry can not be uio-ed

Previosly readdir missed some directory entries because there was
no space for them in current uio but directory stream offset
was advanced nevertheless.
jhb has discoved the issue and provided a test-case.

Reviewed by:	bde
Approved by:	jhb (mentor)

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase