Fix annoying bug with grep and HAMMER

Fix annoying bug with grep and HAMMER

grep foo * on an UFS partition was silent if grep hit a subdirectory.  If
executed on HAMMER, grep complains about "Invalid argument" because directories
in HAMMER are not treated as files.

Before:

cd /usr/src
grep test *
Makefile:       test \
grep: cat: Invalid argument
[...]

After:

grep test *
Makefile:       test \