Rework r252313:

The filedesc lock may not be dropped unconditionally before exporting
fd to sbuf: fd might go away during execution.  While it is ok for
DTYPE_VNODE and DTYPE_FIFO because the export is from a vrefed vnode
here, for other types it is unsafe.

Instead, drop the lock in export_fd_to_sb(), after preparing data in
memory and before writing to sbuf.

Spotted by:	mjg
Suggested by:	kib
Review by:	kib
MFC after:	1 week