fs: fuse: lock the new non-CMA page before replace_page_cache_page()

While swapping a page in FUSE filesystem from a CMA to non-CMA page,
we call replace_page_cache_page() to swap and this function expects
old and new page to be locked. Else we will hit a VM_BUG_ON in
replace_page_cache_page().

Hence, lock the page before calling replace_page_cache_page() to
satisfy its requirement and prevent hitting VM_BUG_ON condition.

CRs-Fixed: 751088
Change-Id: I1dbb90dbaa9f056f211754bad15ae76c9d7171a5
Signed-off-by: Vignesh Radhakrishnan <vigneshr@codeaurora.org>