Replace zed_file_create_dirs() with mkdirp()

Replace zed_file_create_dirs() with mkdirp()

When processing directory components starting from the root dir,
zed_file_create_dirs() contained a bug in checking the return value of
mkdir().  A typo was made, and the test for (mkdir_errno != EEXIST) was
erroneously written as (mkdir_errno == EEXIST).  If some of the leading
directory components already existed, this bug would cause the routine
to exit before creating the remaining directory components.

Instead of fixing the above mkdir_errno test, this commit replaces
zed_file_create_dirs() with mkdirp().  This cleanup was already
planned, and zed_file_create_dirs() only existed because I didn't
realize mkdirp() was already in tree at the time.

Signed-off-by: Chris Dunlap <cdunlap@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2248