mm/vmalloc.c: remove dead code in vb_alloc

Space in a vmap block that was once allocated is considered dirty and
not made available for allocation again before the whole block is
recycled.  The result is that free space within a vmap block is always
contiguous.

So if a vmap block has enough free space for allocation, the allocation
is impossible to fail.  Thus, the fragmented block purging was never
invoked from vb_alloc().  So remove this dead code.

[ Same patches also sent by:

    Chanho Min <chanho.min@lge.com>
    Johannes Weiner <hannes@cmpxchg.org>

  but git doesn't do "multiple authors" ]

Signed-off-by: Zhang Yanfei <zhangyanfei@cn.fujitsu.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>