xfs: make xfs_extent_busy_trim not static

Commit e459df5, 'xfs: move busy extent handling to it's own file'
moved some code from xfs_alloc.c into xfs_extent_busy.c for
convenience in userspace code merges.  One of the functions moved is
xfs_extent_busy_trim (formerly xfs_alloc_busy_trim) which is defined
STATIC.  Unfortunately this function is still used in xfs_alloc.c, and
this results in an undefined symbol in xfs.ko.

Make xfs_extent_busy_trim not static and add its prototype to
xfs_extent_busy.h.

Signed-off-by: Ben Myers <bpm@sgi.com>
Reviewed-by: Mark Tinguely <tinguely@sgi.com>

fs/xfs/xfs_extent_busy.c

@@ -345,7 +345,7 @@ restart:
  * args->minlen no suitable extent could be found, and the higher level
  * code needs to force out the log and retry the allocation.
  */
-STATIC void
+void
 xfs_extent_busy_trim(
 	struct xfs_alloc_arg	*args,
 	xfs_agblock_t		bno,