gfs2: Remove dead code in gfs2_file_read_iter

Function iomap_dio_rw() only returns -ENOTBLK for write requests and
gfs2_file_direct_read() no longer returns -ENOTBLK since commit
1d45bb7f ("gfs2: Use iomap for stuffed direct I/O reads"), so there
is no need to check for -ENOTBLK in gfs2_file_read_iter() anymore.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>