nvmet-file: clamp-down file namespace lba_shift

When the backing file is a tempfile for example, the inode i_blkbits
can be 1M in size which causes problems for hosts to support as the
disk block size. Instead, expose the minimum between i_blkbits and
12 (4K sector size).

Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: - Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>