xfs: clean up locking in xfs_file_iomap_begin

Rather than checking what kind of locking is needed in a helper
function and then jumping through hoops to do the locking in line,
move the locking to the helper function that does all the checks
and rename it to xfs_ilock_for_iomap().

This also allows us to hoist all the nonblocking checks up into the
locking helper, further simplifier the code flow in
xfs_file_iomap_begin() and making it easier to understand.

Signed-Off-By: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>