libfs: Open code simple_commit_write into only user
Boaz Harrosh authored

* simple_commit_write was only called by simple_write_end.
  Open coding it makes it tiny bit less heavy on the arithmetic and
  much more readable.

* While at it use zero_user() for clearing a partial page.
* While at it add a docbook comment for simple_write_end.
Signed-off-by: default avatarBoaz Harrosh <bharrosh@panasas.com>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
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