ext4: clean up len and offset checks in ext4_fallocate()

ext4: clean up len and offset checks in ext4_fallocate()

- Fix some comments.

- Consistently access i_size directly rather than using i_size_read(),
  since in all relevant cases we're under inode_lock().

- Simplify the alignment checks by using the IS_ALIGNED() macro.

- In ext4_insert_range(), do the check against s_maxbytes in a way
  that is safe against signed overflow.  (This doesn't currently matter
  for ext4 due to ext4's limited max file size, but this is something
  other filesystems have gotten wrong.  We might as well do it safely.)

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20191231180444.46586-3-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>