MINOR: fd: Add BUG_ON checks on fd_insert()

MINOR: fd: Add BUG_ON checks on fd_insert()

This patch adds two BUG_ON on fd_insert() into the fdtab checking
if the fd has been correctly re-initialized into the fdtab
before a new insert.

It will raise a BUG if we try to insert the same fd multiple times
without an intermediate fd_delete().

First one checks that the owner for this fd in fdtab was reset to NULL.

Second one checks that the state flags for this fd in fdtab was reset
to 0.

This patch could be backported on version >= 2.4