Factor out some of the duplicated code in the symbol lookup functions, in
preparation for adding userland CTF support to DTrace.

MFC after:	1 month
Sponsored by:	EMC / Isilon Storage Division
reiserfs: Use signed i_nlink

Unlike Linux, FreeBSD's kernel doesn't like unsigned file link
count. This is consistent with our ext2fs implementation.

MFC after:	1 month