ufs: Export UFS_MAXNAMLEN to pathconf, statfs

Rather than the global NAME_MAX constant. This change is required to
support systems with a NAME_MAX/MAXNAMLEN that differs from UFS_MAXNAMLEN.

This was missed in r313475 due to the alternative spelling ("NAME_MAX") of
MAXNAMLEN. This change is also similar in spirit to r313780.

Reported by: ngie@
Sponsored by: Dell EMC Isilon