Reduce code duplication, introduce the getmaxfd() helper to calculate
the max filedescriptor index.

Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (marius)
When growing the file descriptor table, new larger memory chunk is
allocated, but the old table is kept around to handle the case of
threads still performing unlocked accesses to it.

Grow the table exponentially instead of increasing its size by
sizeof(long) * 8 chunks when overflowing. This mode significantly
reduces the total memory use for the processes consuming large numbers
of the file descriptors which open them one by one.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (marius)
Do not flush buffers when the v_object of the passed vnode does not
really belong to it. Such vnodes, with the pointers to other vnodes
v_objects, are typically instantiated by the bypass filesystems.
Invalidating mappings of other vnode pages and the pages is wrong,
since reclamation of the upper vnode does not imply that lower vnode
is reclaimed too.

One of the consequences of the improper reclamation was destruction of
the wired mappings of the lower vnode pages, triggering miscellaneous
assertions in the VM system.

Reported by:    John Marshall <john.marshall@riverwillow.com.au>
Tested by:      John Marshall <john.marshall@riverwillow.com.au>, pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (gjb)
The device vnodes are often unlocked when bread() or bwrite() is
called.  This probably should be fixed eventually, but for now it is
not needed to try to flush such vnodes from the buffer allocation
context.

Reported and tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Approved by:	re (gjb)