ocfs2: Fix ocfs2_page_mkwrite()

This patch address two shortcomings in ocfs2_page_mkwrite():
1. Makes the function return better VM_FAULT_* errors.
2. It handles a error that is triggered when a page is dropped from the mapping
due to memory pressure. This patch locks the page to prevent that.

[Patch was cleaned up by Sunil Mushran.]

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Sunil Mushran <sunil.mushran@oracle.com>