vfs: remove unused wrapper block_page_mkwrite()

The function currently called "__block_page_mkwrite()" used to be called
"block_page_mkwrite()" until a wrapper for this function was added by:

commit 24da4fab5a61 ("vfs: Create __block_page_mkwrite() helper passing
	error values back")

This wrapper, the current "block_page_mkwrite()", is currently unused.
__block_page_mkwrite() is used directly by ext4, nilfs2 and xfs.

Remove the unused wrapper, rename __block_page_mkwrite() back to
block_page_mkwrite() and update the comment above block_page_mkwrite().

Signed-off-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Reviewed-by: Jan Kara <jack@suse.com>
Cc: Jan Kara <jack@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 7d1aad1..ea433a7 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -5283,7 +5283,7 @@
 	    !ext4_should_journal_data(inode) &&
 	    !ext4_nonda_switch(inode->i_sb)) {
 		do {
-			ret = __block_page_mkwrite(vma, vmf,
+			ret = block_page_mkwrite(vma, vmf,
 						   ext4_da_get_block_prep);
 		} while (ret == -ENOSPC &&
 		       ext4_should_retry_alloc(inode->i_sb, &retries));
@@ -5330,7 +5330,7 @@
 		ret = VM_FAULT_SIGBUS;
 		goto out;
 	}
-	ret = __block_page_mkwrite(vma, vmf, get_block);
+	ret = block_page_mkwrite(vma, vmf, get_block);
 	if (!ret && ext4_should_journal_data(inode)) {
 		if (ext4_walk_page_buffers(handle, page_buffers(page), 0,
 			  PAGE_CACHE_SIZE, NULL, do_journal_get_write_access)) {