ext4: do not block RWF_NOWAIT dio write on unallocated space

ext4: do not block RWF_NOWAIT dio write on unallocated space

Since commit 0cfa92af90d7 ("ext4: introduce direct I/O write using iomap
infrastructure") we don't properly bail out of RWF_NOWAIT direct IO
write if underlying blocks are not allocated. Also
ext4_dio_write_checks() does not honor RWF_NOWAIT when re-acquiring
i_rwsem. Fix both issues.

Fixes: 0cfa92af90d7 ("ext4: introduce direct I/O write using iomap infrastructure")
Cc: stable@kernel.org
Reported-by: Filipe Manana <fdmanana@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200708153516.9507-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>