staging: lustre: mdc: remove console spew from mdc_ioc_fid2path
In some cases with a very long pathname, such as with sanity.sh
test_154c, mdc_ioc_fid2path() would spew long debug messages to
the log, because libcfs_debug_vmsg2() refuses to log messages over
one page in size.
Truncate the debug message to only log the last 512 characters
of the pathname, which is sufficient for most debugging, saves a
bit of space in the debug log, and will prevent the debug logging
from printing to the console in the first place.