iomap: fix WARN_ON_ONCE on uninitialized variable

In commit 9dc55f13 ("iomap: add support for sub-pagesize buffered
I/O without buffer heads") we moved the initialization of poff (it's
computed from pos) into a separate helper function.  Inline data only
ever deals with pos == 0, hence the WARN_ON_ONCE, but now we're testing
an uninitialized variable.

Therefore, change the test to check the parameter directly.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>