f2fs: fix to update iostat correctly in f2fs_filemap_fault()

In f2fs_filemap_fault(), it fixes to update iostat info only if
VM_FAULT_LOCKED is tagged in return value of filemap_fault().

Fixes: 8b83ac81

 ("f2fs: support read iostat")
Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>