libstand: Reset the seek pointer in ext2fs as done in UFS.

Based on r134760:

Reset the seek pointer to 0 when a file is successfully opened,
since otherwise the initial seek offset will contain the directory
offset of the filesystem block that contained its directory entry.
This bug was mostly harmless because typically the directory is
less than one filesystem block in size so the offset would be zero.
It did however generally break loading a kernel from the (large)
kernel compile directory.

Also reset the seek pointer when a new inode is opened in read_inode(),
though this is not actually necessary now because all callers set
it afterwards.

PR:		177328
Submitted by:	Eric van Gyzen
Reviewed by:	iedowse
MFC after:	5 days