xfs: Add xfs_iget() to filesystem ops

xfs: Add xfs_iget() to filesystem ops

xfs_iget() function is responsible for finding an entry from a given
filename and parent inode of the entry being looked up.

XFS_INO_TO_FSB() returns a sparse enconding of the disk location, and not
the right filesystem block number we expected to have from the wrong byte
offset.

Now, there is a ino_to_bytes() macro who calculates correctly the block
offset from a given inode number, and then we get the *right* filesystem
block number we expected.

Also, use agnumber_to_bytes() in order to get the right filesystem block
where the given AG number resides.

Signed-off-by: Paulo Alcantara <pcacjr@zytor.com>