Fix maybe uninitialized variable warning

Commit 1c2358c12 restructured this code and introduced a warning
about the variable maybe not being initialized. This cannot happen
with the updated code but we should initialize the variable anyway
to silence the warning.

    zpl_file.c: In function ‘zpl_iter_write’:
    zpl_file.c:324:9: warning: ‘count’ may be used uninitialized
        in this function [-Wmaybe-uninitialized]

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #11373