Set f_ncp in the struct file unconditionally. Previously we only set it

Set f_ncp in the struct file unconditionally.  Previously we only set it
when opening directories.  This allows the f*() system calls such as
fchmod() to check the actual mount point instead of the aliased mount
point (in the case of a NULLFS mount).  Also, the fstat program will
properly report the path for descriptors opened via nullfs mounts.

Add code to all f*() system calls such as fchmod() to check f_ncp
in order to detect read-only nullfs mounts.