ceph: reset i_requested_max_size if file write is not wanted

write can stuck at waiting for larger max_size in following sequence of
events:

- client opens a file and writes to position 'A' (larger than unit of
  max size increment)
- client closes the file handle and updates wanted caps (not wanting
  file write caps)
- client opens and truncates the file, writes to position 'A' again.

At the 1st event, client set inode's requested_max_size to 'A'. At the
2nd event, mds removes client's writable range, but client does not reset
requested_max_size. At the 3rd event, client does not request max size
because requested_max_size is already larger than 'A'.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>