fd: Avoid truncating output buffers for KERN_PROC_{CWD,FILEDESC}

These sysctls failed to return an error if the caller had provided too
short an output buffer. Change them to return ENOMEM instead, to ensure
that callers can detect truncation in the face of a concurrently
changing fd table.

PR: 228432
Discussed with: cem, jhb
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D15607