[patch] new aop block_write_begin fix

[Dmitriy Monakhov <dmonakhov@sw.ru>]

	After we enter to block_write_begin with *pagep == NULL and
 	some page was grabed we remember this page in *pagep
 	And if __block_prepare_write() we have to clear *pagep , as 
 	it was before. Because this may confuse caller.
 	for example caller may have folowing code:
 		ret = block_write_begin(..., pagep,...)
 		if (ret && *pagep != NULL) {
 			unlock_page(*pagep);
 			page_cache_release(*pagep);
 		}
Signed-off-by: Dmitriy Monakhov <dmonakhov@openvz.org>

diff --git a/fs/buffer.c b/fs/buffer.c
index 07cd457..df933ba 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -1982,6 +1982,7 @@ int block_write_begin(struct file *file, struct address_space *mapping,
 		if (ownpage) {
 			unlock_page(page);
 			page_cache_release(page);
+			*pagep = NULL;
 
 			/*
 			 * prepare_write() may have instantiated a few blocks