fs: mark __generic_block_fiemap static

There is no caller left outside of ioctl.c.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Link: https://lore.kernel.org/r/20200523073016.2944131-4-hch@lst.de


Signed-off-by: Theodore Ts'o <tytso@mit.edu>