NFSv4.1: Remove a bogus BUG_ON() in nfs4_layoutreturn_done

NFSv4.1: Remove a bogus BUG_ON() in nfs4_layoutreturn_done

Ever since commit 0a57cdac3f (NFSv4.1 send layoutreturn to fence
disconnected data server) we've been sending layoutreturn calls
while there is potentially still outstanding I/O to the data
servers. The reason we do this is to avoid races between replayed
writes to the MDS and the original writes to the DS.

When this happens, the BUG_ON() in nfs4_layoutreturn_done can
be triggered because it assumes that we would never call
layoutreturn without knowing that all I/O to the DS is
finished. The fix is to remove the BUG_ON() now that the
assumptions behind the test are obsolete.

Reported-by: Boaz Harrosh <[email protected]>
Reported-by: Tigran Mkrtchyan <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>
Cc: [email protected] [>=3.5]