nfs_readlink() calls nfs_bioread() which passes in uio_td as the thread
argument to nfs_getcacheblk(). In nfs_getcacheblk() we dereference the
thread pointer to get a process pointer to pass to nfs_sigintr(). This
obviously results in a panic. :)
Rather than change nfs_getcacheblk() to check if the thread pointer is
NULL when calling nfs_sigintr() like other callers do, change
nfs_sigintr() to take a thread as the last argument instead of a
process so none of the callers have to care if the thread is NULL or not.