Revert change in revision 1.53 and add a small comment to protect the revived code.
Revert change in revision 1.53 and add a small comment to protect
the revived code.

vm pages newly allocated are marked busy (PG_BUSY), thus calling
vm_page_delete before the pages has been freed or unbusied will
cause a deadlock since vm_page_object_page_remove will wait for the
busy flag to be cleared.  This can be triggered by calling malloc
with size > PAGE_SIZE and the M_NOWAIT flag on systems low on
physical free memory.

A kernel module that reproduces the problem, written by Logan Gabriel
<logan@mail.2cactus.com>, can be found in the freebsd-hackers mail
archive (12 Apr 2001).  The problem was recently noticed again by
Archie Cobbs <archie@dellroad.org>.

Reviewed by:	dillon

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