NFS: Don't loop forever in nfs_do_recoalesce()

If __nfs_pageio_add_request() fails to add the request, it will return
with either desc->pg_error < 0, or mirror->pg_recoalesce will be set, so
we are guaranteed either to exit the function altogether, or to loop.

However if there is nothing left in mirror->pg_list to coalesce, we must
exit, so make sure that we clear mirror->pg_recoalesce every time we
loop.

Reported-by: Olga Kornievskaia <aglo@umich.edu>
Fixes: 70536bf4 ("NFS: Clean up reset of the mirror accounting variables")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>